SWIG and PHP

# 34 SWIG and PHP

- Generating PHP Extensions
  - Building a loadable extension
  - Using PHP Extensions
- Basic PHP interface
  - Constants
  - Global Variables
  - Functions
  - Overloading
  - Pointers and References
  - Structures and C++ classes
    - Using `-noproxy`
    - Constructors and Destructors
    - Static Member Variables
    - Static Member Functions
    - Specifying Implemented Interfaces
  - PHP Pragmas, Startup and Shutdown code
- Cross language polymorphism
  - Enabling directors
  - Director classes
  - Ownership and object destruction
  - Exception unrolling
  - Overhead and code bloat
  - Typemaps
  - Miscellaneous

SWIG supports generating wrappers for PHP5. Support for PHP4 was removed in SWIG 1.3.37. The PHP developers are no longer making new PHP4 releases, and won't even be patching critical security issues after 2008-08-08, so it doesn't make much sense for SWIG to continue to support PHP4 now. If you really need to continue to use PHP4, just stick with SWIG 1.3.36.

Currently any PHP5 release should work, but we don't regularly test with PHP < 5.3.

In this chapter, we discuss SWIG's support of PHP. The PHP module was extensively rewritten in release 1.3.26, and support for generating OO wrappers for PHP5 was added in 1.3.30. The PHP module now supports most of the features available in some of the other languages.

In order to use this module, you will need to have a copy of the PHP5 include files to compile the SWIG generated files. If you installed PHP from a binary package, you may need to install a "php-dev" or "php-devel" package for these to be installed. You can find out where these files are by running `php-config --includes`. To use the built PHP module you will need either the php binary or the Apache php module. If you want to build your extension into php directly, you will need the complete PHP source tree available.

## 34.1 Generating PHP Extensions

To build a PHP extension, run swig using the `-php` option as follows:

    swig -php example.i

This will produce 3 files example_wrap.c, php_example.h and example.php. The first file, `example_wrap.c` contains all of the C code needed to build a PHP extension. The second file, `php_example.h` contains the header information needed if you wish to statically link the extension into the php interpreter. The third file, `example.php` can be included by PHP scripts. It attempts to dynamically load the extension and contains extra php code specified in the interface file. If wrapping C++ code with PHP classes, it will also contain PHP5 class wrappers.

SWIG can generate PHP extensions from C++ libraries as well when given the `-c++` option. The support for C++ is discussed in more detail in section 27.2.6.

The usual (and recommended) way is to build the extension as a separate dynamically loaded module (which is supported by all modern operating systems).

It is also possible to rebuild PHP from source so that your module is statically linked into the php executable/library. This is a lot more work, and also requires a full rebuild of PHP to update your module, and it doesn't play nicely with package system. We don't recommend this approach, or provide explicit support for it.

### 34.1.1 Building a loadable extension

To build your module as a dynamically loadable extension, use compilation commands like these (if you aren't using GCC, the commands will be different, and there may be some variation between platforms - these commands should at least work for Linux though):

    	gcc `php-config --includes` -fpic -c example_wrap.c example.c
    	gcc -shared example_wrap.o example.o -o example.so

### 34.1.2 Using PHP Extensions

To test the extension from a PHP script, you first need to tell PHP to load it. To do this, add a line like this to the `[PHP]` section of `php.ini`:

    	extension=/path/to/modulename.so

If the module is in PHP's default extension directory, you can omit the path.

For some SAPIs (for example, the CLI SAPI) you can instead use the dl() function to load an extension at run time, by adding a like like this to the start of each PHP script which uses your extension:

    	dl("/path/to/modulename.so");	// Load the module

But note that this doesn't work when running PHP through a webserver in PHP5.3 and later - you'll need to use `extension` in `php.ini` as described above.

The PHP module which SWIG generates will also attempt to do the `dl()` call for you if the extension isn't already loaded:

    	include("example.php");

This PHP module also defines the PHP classes for the wrapped API, so you'll almost certainly want to include it anyway.

## 34.2 Basic PHP interface

It is important to understand that PHP uses a single global namespace into which all symbols from extension modules are loaded. It is quite possible for names of symbols in one extension module to clash with other symbols unless care is taken to `%rename` them. At present SWIG doesn't have support for the namespace feature added in PHP 5.3.

### 34.2.1 Constants

These work in much the same way as in C/C++. Constants can be defined by using either the normal C pre-processor declarations, or the `%constant` SWIG directive. These will then be available from your PHP script as a PHP constant, (i.e. no dollar sign is needed to access them.) For example, with a swig interface file like this,

    %module example

    #define PI 3.14159

    %constant int E  = 2.71828

you can access the constants in your PHP script like this,

    include("example.php");

    echo "PI = " . PI . "\n";

    echo "E = " . E . "\n";

There's one peculiarity of how constants work in PHP which it is useful to note (this is not specific to SWIG though) - if you try to use an undeclared constant, PHP will emit a notice and then expand the constant to a string version of the constant's name. Unfortunately it is easy to miss the notice if you're using PHP in a webserver, as it will probably end up in error.log or similar.

For example,

    %module example

    #define EASY_TO_MISPELL	0

accessed incorrectly in PHP,

    include("example.php");

    if(EASY_TO_MISPEL) {
    	....
    } else {
    	....
    }

The mis-spelled constant will become the string 'EASY_TO_MISPEL', which is treated as true by the if test, when the value of the intended constant would be treated as false!

### 34.2.2 Global Variables

Because PHP does not provide a mechanism to intercept access and assignment of global variables, global variables are supported through the use of automatically generated accessor functions.

    %module example;

    %inline %{
      double seki = 2;
      void print_seki() {
        zend_printf("seki is now %f\n",seki);
      }
    %}

is accessed as follows:

    include("example.php");
    print seki_get();
    seki_set( seki_get() * 2);	# The C variable is now 4.
    print seki_get();

SWIG supports global variables of all C datatypes including pointers and complex objects. Additional types can be supported by using the `varinit` typemap.

SWIG honors the `%immutable` modifier by not generating code for the `_set` method. This provides read-only access to the variable from the php script. Attempting to access the `_set` method will result in a php fatal error because the function is undefined.

At this time SWIG does not support custom accessor methods.

### 34.2.3 Functions

C functions are converted into PHP functions. Default/optional arguments are also allowed. An interface file like this :

    %module example
    int foo(int a);
    double bar(double, double b = 3.0);
    ...

Will be accessed in PHP like this :

    include("example.php");
    $a = foo(2);
    $b = bar(3.5, -1.5);
    $c = bar(3.5);		# Use default argument for 2nd parameter

### 34.2.4 Overloading

Although PHP does not support overloading functions natively, swig will generate dispatch functions which will use `%typecheck` typemaps to allow overloading. This dispatch function's operation and precedence is described in Wrapping Overloaded Functions and Methods.

### 34.2.5 Pointers and References

Pointers to C/C++ objects are represented as PHP resources, rather like MySQL connection handles.

There are multiple ways to wrap pointers to simple types. Given the following C method:

      void add( int *in1, int *in2, int *result);

One can include cpointer.i to generate PHP wrappers to `int *`.

    %module example
    %include "cpointer.i"
    %pointer_functions(int,intp)

    void add( int *in1, int *in2, int *result);

This will result in the following usage in PHP:

    <?php

    include("example.php");

    $in1=copy_intp(3);
    $in2=copy_intp(5);
    $result=new_intp();

    add( $in1, $in2, $result );

    echo "The sum " . intp_value($in1) . " + " . intp_value($in2) . " = " . intp_value( $result) . "\n";
    ?>

An alternative would be to use the include typemaps.i which defines named typemaps for INPUT, OUTPUT and INOUT variables. One needs to either `%apply` the appropriate typemap or adjust the parameter names as appropriate.

    %module example
    %include "typemaps.i"

    void add( int *INPUT, int *INPUT, int *OUTPUT);

This will result in the following usage in PHP:

    <?php

    include("example.php");

    $in1 = 3;
    $in2 = 5;
    $result= add($in1,$in2);  # Note using variables for the input is unnecessary.

    echo "The sum $in1 + $in2 = $result\n";
    ?>

Because PHP has a native concept of reference, it may seem more natural to the PHP developer to use references to pass pointers. To enable this, one needs to include phppointers.i which defines the named typemap REF.

Prior to SWIG 3.0, the REF typemaps relied on PHP's call-time pass-by-reference, which was deprecated in PHP 5.3 and removed in PHP 5.4. So if you use these REF typemaps, you should ensure that SWIG≥3.0 is used to generate wrappers from your interface file.

In case you write your own typemaps, SWIG supports an attribute called `byref`: if you set that, then SWIG will make sure that the generated wrapper function will want the input parameter as a reference.

    %module example
    %include "phppointers.i"

    void add( int *REF, int *REF, int *REF);

This will result in the following usage in PHP:

    <?php

    include("example.php");

    $in1 = 3;
    $in2 = 5;
    $result = 0;
    add($in1,$in2,$result);

    echo "The sum $in1 + $in2 = $result\n";
    ?>

It is important to note that a php variable which is NULL when passed by reference would end up passing a NULL pointer into the function. In PHP, an unassigned variable (i.e. where the first reference to the variable is not an assignment) is NULL. In the above example, if any of the three variables had not been assigned, a NULL pointer would have been passed into `add`. Depending on the implementation of the function, this may or may not be a good thing.

We chose to allow passing NULL pointers into functions because that is sometimes required in C libraries. A NULL pointer can be created in PHP in a number of ways: by using `unset` on an existing variable, or assigning `NULL` to a variable.

### 34.2.6 Structures and C++ classes

SWIG defaults to wrapping C++ structs and classes with PHP classes - this is done by generating a PHP wrapper script which defines proxy classes which calls a set of flat functions which actually wrap the C++ class. You can disable this wrapper layer by passing the command-line option "-noproxy" in which case you'll just get the flat functions.

This interface file

    %module vector

    class Vector {
    public:
    	double x,y,z;
    	Vector();
    	~Vector();
    	double magnitude();
    };

    struct Complex {
     double re, im;
    };

Would be used in the following way from PHP5:

    <?php
      require "vector.php";

      $v = new Vector();
      $v->x = 3;
      $v->y = 4;
      $v->z = 5;

      echo "Magnitude of ($v->x,$v->y,$v->z) = " . $v->magnitude() . "\n";

      $v = NULL;   # destructor called.

      $c = new Complex();

      $c->re = 0;
      $c->im = 0;

      # $c destructor called when $c goes out of scope.
    ?>

Member variables and methods are accessed using the `->` operator.

#### 34.2.6.1 Using `-noproxy`

The `-noproxy` option flattens the object structure and generates collections of named functions (these are the functions which the PHP5 class wrappers call). The above example results in the following PHP functions:

    new_Vector();
    Vector_x_set($obj,$d);
    Vector_x_get($obj);
    Vector_y_set($obj,$d);
    Vector_y_get($obj);
    Vector_z_set($obj,$d);
    Vector_z_get($obj);
    Vector_magnitude($obj);
    new_Complex();
    Complex_re_set($obj,$d);
    Complex_re_get($obj);
    Complex_im_set($obj,$d);
    Complex_im_get($obj);

#### 34.2.6.2 Constructors and Destructors

The constructor is called when `new Object()` (or `new_Object()` if using `-noproxy`) is used to create an instance of the object. If multiple constructors are defined for an object, function overloading will be used to determine which constructor to execute.

Because PHP uses reference counting to manage resources, simple assignment of one variable to another such as:

    $ref = $v;

causes the symbol `$ref` to refer to the same underlying object as `$v`. This does not result in a call to the C++ copy constructor or copy assignment operator.

One can force execution of the copy constructor by using:

    $o_copy = new Object($o);

Destructors are automatically called when all variables referencing the instance are reassigned or go out of scope. The destructor is not available to be called manually. To force a destructor to be called the programmer can either reassign the variable or call `unset($v)`

#### 34.2.6.3 Static Member Variables

Static member variables in C++ are not wrapped as such in PHP as it does not appear to be possible to intercept accesses to such variables. Therefore, static member variables are wrapped using a class function with the same name, which returns the current value of the class variable. For example

    %module example

    class Ko {
    	static int threats;
    };

would be accessed in PHP as,

    include("example.php");

    echo "There have now been " . Ko::threats() . " threats\n";

To set the static member variable, pass the value as the argument to the class function, e.g.

    Ko::threats(10);

    echo "There have now been " . Ko::threats() . " threats\n";

#### 34.2.6.4 Static Member Functions

Static member functions are supported in PHP using the `class::function()` syntax. For example

    %module example
    class Ko {
      static void threats();
    };

would be executed in PHP as,

    include("example.php");
    Ko::threats();

#### 34.2.6.5 Specifying Implemented Interfaces

PHP supports the concept of abstract interfaces which a class can implement. Since SWIG 3.0.3, you can tell SWIG that a wrapped class (for example `MyIterator`) implements the `Iterator` interface like so:

    %typemap("phpinterfaces") MyIterator "Iterator";

If there are multiple interfaces, just list them separated by commas.

### 34.2.7 PHP Pragmas, Startup and Shutdown code

To place PHP code in the generated "example.php" file one can use the code pragma. The code is inserted after loading the shared object.

    %module example
    %pragma(php) code="
    # This code is inserted into example.php
    echo \"example.php execution\\n\";
    "

Results in the following in "example.php"

    # This code is inserted into example.php
    echo "example.php execution\n";

The include pragma is a short cut to add include statements to the example.php file.

    %module example
    %pragma(php) code="
    include \"include.php\";
    "
    %pragma(php) include="include.php"   // equivalent.

The phpinfo pragma inserts code in the `PHP_MINFO_FUNCTION` which is called from PHP's phpinfo() function.

    %module example;
    %pragma(php) phpinfo="
      zend_printf("An example of PHP support through SWIG\n");
      php_info_print_table_start();
      php_info_print_table_header(2, \"Directive\", \"Value\");
      php_info_print_table_row(2, \"Example support\", \"enabled\");
      php_info_print_table_end();
    "

To insert code into the `PHP_MINIT_FUNCTION`, one can use either `%init` or `%minit`.

    %module example;
    %init {
      zend_printf("Inserted into PHP_MINIT_FUNCTION\n");
    }
    %minit {
      zend_printf("Inserted into PHP_MINIT_FUNCTION\n");
    }

To insert code into the `PHP_MSHUTDOWN_FUNCTION`, one can use either `%shutdown` or `%mshutdown`.

    %module example;
    %mshutdown {
      zend_printf("Inserted into PHP_MSHUTDOWN_FUNCTION\n");
    }

The `%rinit` and `%rshutdown` statements are very similar but insert code into the request init (PHP_RINIT_FUNCTION) and request shutdown (PHP_RSHUTDOWN_FUNCTION) code respectively.

## 34.3 Cross language polymorphism

Proxy classes provide a more natural, object-oriented way to access extension classes. As described above, each proxy instance has an associated C++ instance, and method calls to the proxy are passed to the C++ instance transparently via C wrapper functions.

This arrangement is asymmetric in the sense that no corresponding mechanism exists to pass method calls down the inheritance chain from C++ to PHP. In particular, if a C++ class has been extended in PHP (by extending the proxy class), these extensions will not be visible from C++ code. Virtual method calls from C++ are thus not able access the lowest implementation in the inheritance chain.

Changes have been made to SWIG 1.3.18 to address this problem and make the relationship between C++ classes and proxy classes more symmetric. To achieve this goal, new classes called directors are introduced at the bottom of the C++ inheritance chain. Support for generating PHP classes has been added in SWIG 1.3.40. The job of the directors is to route method calls correctly, either to C++ implementations higher in the inheritance chain or to PHP implementations lower in the inheritance chain. The upshot is that C++ classes can be extended in PHP and from C++ these extensions look exactly like native C++ classes. Neither C++ code nor PHP code needs to know where a particular method is implemented: the combination of proxy classes, director classes, and C wrapper functions takes care of all the cross-language method routing transparently.

### 34.3.1 Enabling directors

The director feature is disabled by default. To use directors you must make two changes to the interface file. First, add the "directors" option to the %module directive, like this:

    %module(directors="1") modulename

Without this option no director code will be generated. Second, you must use the %feature("director") directive to tell SWIG which classes and methods should get directors. The %feature directive can be applied globally, to specific classes, and to specific methods, like this:

    // generate directors for all classes that have virtual methods
    %feature("director");

    // generate directors for all virtual methods in class Foo
    %feature("director") Foo;

You can use the %feature("nodirector") directive to turn off directors for specific classes or methods. So for example,

    %feature("director") Foo;
    %feature("nodirector") Foo::bar;

will generate directors for all virtual methods of class Foo except bar().

Directors can also be generated implicitly through inheritance. In the following, class Bar will get a director class that handles the methods one() and two() (but not three()):

    %feature("director") Foo;
    class Foo {
    public:
        Foo(int foo);
        virtual void one();
        virtual void two();
    };

    class Bar: public Foo {
    public:
        virtual void three();
    };

then at the PHP side you can define

    require("mymodule.php");

    class MyFoo extends Foo {
      function one() {
         print "one from php\n";
      }
    }

### 34.3.2 Director classes

For each class that has directors enabled, SWIG generates a new class that derives from both the class in question and a special `Swig::Director` class. These new classes, referred to as director classes, can be loosely thought of as the C++ equivalent of the PHP proxy classes. The director classes store a pointer to their underlying PHP object. Indeed, this is quite similar to the "_cPtr" and "thisown" members of the PHP proxy classes.

For simplicity let's ignore the `Swig::Director` class and refer to the original C++ class as the director's base class. By default, a director class extends all virtual methods in the inheritance chain of its base class (see the preceding section for how to modify this behavior). Thus all virtual method calls, whether they originate in C++ or in PHP via proxy classes, eventually end up in at the implementation in the director class. The job of the director methods is to route these method calls to the appropriate place in the inheritance chain. By "appropriate place" we mean the method that would have been called if the C++ base class and its extensions in PHP were seamlessly integrated. That seamless integration is exactly what the director classes provide, transparently skipping over all the messy extension API glue that binds the two languages together.

In reality, the "appropriate place" is one of only two possibilities: C++ or PHP. Once this decision is made, the rest is fairly easy. If the correct implementation is in C++, then the lowest implementation of the method in the C++ inheritance chain is called explicitly. If the correct implementation is in PHP, the Zend API is used to call the method of the underlying PHP object (after which the usual virtual method resolution in PHP automatically finds the right implementation).

Now how does the director decide which language should handle the method call? The basic rule is to handle the method in PHP, unless there's a good reason not to. The reason for this is simple: PHP has the most "extended" implementation of the method. This assertion is guaranteed, since at a minimum the PHP proxy class implements the method. If the method in question has been extended by a class derived from the proxy class, that extended implementation will execute exactly as it should. If not, the proxy class will route the method call into a C wrapper function, expecting that the method will be resolved in C++. The wrapper will call the virtual method of the C++ instance, and since the director extends this the call will end up right back in the director method. Now comes the "good reason not to" part. If the director method were to blindly call the PHP method again, it would get stuck in an infinite loop. We avoid this situation by adding special code to the C wrapper function that tells the director method to not do this. The C wrapper function compares the called and the declaring class name of the given method. If these are not the same, then the C wrapper function tells the director to resolve the method by calling up the C++ inheritance chain, preventing an infinite loop.

One more point needs to be made about the relationship between director classes and proxy classes. When a proxy class instance is created in PHP, SWIG creates an instance of the original C++ class and assigns it to `->_cPtr`. This is exactly what happens without directors and is true even if directors are enabled for the particular class in question. When a class derived from a proxy class is created, however, SWIG then creates an instance of the corresponding C++ director class. The reason for this difference is that user-defined subclasses may override or extend methods of the original class, so the director class is needed to route calls to these methods correctly. For unmodified proxy classes, all methods are ultimately implemented in C++ so there is no need for the extra overhead involved with routing the calls through PHP.

### 34.3.3 Ownership and object destruction

Memory management issues are slightly more complicated with directors than for proxy classes alone. PHP instances hold a pointer to the associated C++ director object, and the director in turn holds a pointer back to the PHP object. By default, proxy classes own their C++ director object and take care of deleting it when they are garbage collected.

This relationship can be reversed by calling the special `->thisown` property of the proxy class. After setting this property to `0`, the director class no longer destroys the PHP object. Assuming no outstanding references to the PHP object remain, the PHP object will be destroyed at the same time. This is a good thing, since directors and proxies refer to each other and so must be created and destroyed together. Destroying one without destroying the other will likely cause your program to segfault.

Here is an example:

    class Foo {
    public:
        ...
    };
    class FooContainer {
    public:
        void addFoo(Foo *);
        ...
    };

    $c = new FooContainer();
    $a = new Foo();
    $a->thisown = 0;
    $c->addFoo($a);

In this example, we are assuming that FooContainer will take care of deleting all the Foo pointers it contains at some point.

### 34.3.4 Exception unrolling

With directors routing method calls to PHP, and proxies routing them to C++, the handling of exceptions is an important concern. By default, the directors ignore exceptions that occur during method calls that are resolved in PHP. To handle such exceptions correctly, it is necessary to temporarily translate them into C++ exceptions. This can be done with the %feature("director:except") directive. The following code should suffice in most cases:

    %feature("director:except") {
        if ($error == FAILURE) {
            throw Swig::DirectorMethodException();
        }
    }

This code will check the PHP error state after each method call from a director into PHP, and throw a C++ exception if an error occurred. This exception can be caught in C++ to implement an error handler. Currently no information about the PHP error is stored in the Swig::DirectorMethodException object, but this will likely change in the future.

It may be the case that a method call originates in PHP, travels up to C++ through a proxy class, and then back into PHP via a director method. If an exception occurs in PHP at this point, it would be nice for that exception to find its way back to the original caller. This can be done by combining a normal %exception directive with the `director:except` handler shown above. Here is an example of a suitable exception handler:

    %exception {
        try { $action }
        catch (Swig::DirectorException &e) { SWIG_fail; }
    }

The class Swig::DirectorException used in this example is actually a base class of Swig::DirectorMethodException, so it will trap this exception. Because the PHP error state is still set when Swig::DirectorMethodException is thrown, PHP will register the exception as soon as the C wrapper function returns.

### 34.3.5 Overhead and code bloat

Enabling directors for a class will generate a new director method for every virtual method in the class' inheritance chain. This alone can generate a lot of code bloat for large hierarchies. Method arguments that require complex conversions to and from target language types can result in large director methods. For this reason it is recommended that you selectively enable directors only for specific classes that are likely to be extended in PHP and used in C++.

Compared to classes that do not use directors, the call routing in the director methods does add some overhead. In particular, at least one dynamic cast and one extra function call occurs per method call from PHP. Relative to the speed of PHP execution this is probably completely negligible. For worst case routing, a method call that ultimately resolves in C++ may take one extra detour through PHP in order to ensure that the method does not have an extended PHP implementation. This could result in a noticeable overhead in some cases.

Although directors make it natural to mix native C++ objects with PHP objects (as director objects) via a common base class pointer, one should be aware of the obvious fact that method calls to PHP objects will be much slower than calls to C++ objects. This situation can be optimized by selectively enabling director methods (using the %feature directive) for only those methods that are likely to be extended in PHP.

### 34.3.6 Typemaps

Typemaps for input and output of most of the basic types from director classes have been written. These are roughly the reverse of the usual input and output typemaps used by the wrapper code. The typemap operation names are 'directorin', 'directorout', and 'directorargout'. The director code does not currently use any of the other kinds of typemaps. It is not clear at this point which kinds are appropriate and need to be supported.

### 34.3.7 Miscellaneous

Director typemaps for STL classes are mostly in place, and hence you should be able to use std::string, etc., as you would any other type.