# 29 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
  - PHP Pragmas, Startup and Shutdown code

Caution: This chapter (and module!) is still under construction

SWIG supports generating wrappers for PHP5. Support for PHP4 has been removed as of 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 at this point. If you need to continue to use PHP4, stick with SWIG 1.3.36.

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 works fairly well, but currently does not implement all 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.

## 29.1 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). You can then specify that this be loaded automatically in `php.ini` or load it explicitly for any script which needs it.

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.

### 29.1.1 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
    	gcc -shared example_wrap.o -o example.so

### 29.1.2 Using PHP Extensions

To test the extension from a PHP script, you need to load it first. You can load it for every script by adding this line the `[PHP]` section of `php.ini`:

    	extension=/path/to/modulename.so

Alternatively, you can load it explicitly only for scripts which need it by adding this line:

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

to the start of each PHP file. SWIG also generates a php module, which attempts to do the `dl()` call for you:

    	include("example.php");

## 29.2 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.

### 29.2.1 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 are two peculiarities with using constants in PHP. The first is that if you try to use an undeclared constant, it will evaluate to a string set to the constant's name. For example,

    %module example

    #define EASY_TO_MISPELL	0

accessed incorrectly in PHP,

    include("example.php");

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

will issue a warning about the undeclared constant, but will then evaluate it and turn it into a string ('EASY_TO_MISPEL'), which evaluates to true, rather than the value of the constant which would be false. This is a feature!

The second 'feature' is that although constants are case sensitive (by default), you cannot declare a constant twice with alternative cases. E.g.,

    %module example

    #define TEST	Hello
    #define Test	World

accessed from PHP,

    include("example.php");

    echo TEST, Test;

will output "Hello Test" rather than "Hello World". This is because internally, all constants are stored in a hash table by their lower case name, so 'TEST' and 'Test' will map to the same hash element ('Test'). But, because we declared them case sensitive, the Zend engine will test if the case matches with the case the constant was declared with first.

So, in the example above, the TEST constant was declared first, and will be stored under the hash element 'test'. The 'Test' constant will also map to the same hash element 'test', but will not overwrite it. When called from the script, the TEST constant will again be mapped to the hash element 'test' so the constant will be retrieved. The case will then be checked, and will match up, so the value ('Hello') will be returned. When 'Test' is evaluated, it will also map to the same hash element 'test'. The same constant will be retrieved, this time though the case check will fail as 'Test' != 'TEST'. So PHP will assume that Test is a undeclared constant, and as explained above, will return it as a string set to the constant name ('Test'). Hence the script above will print 'Hello Test'. If they were declared non-case sensitive, the output would be 'Hello Hello', as both point to the same value, without the case test taking place. ( Apologies, this paragraph needs rewriting to make some sense. )

### 29.2.2 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.

### 29.2.3 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

### 29.2.4 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.

### 29.2.5 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 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.

### 29.2.6 Structures and C++ classes

SWIG defaults to wrapping C++ structs and classes with PHP classes unless "-noproxy" is specified. For PHP5, a PHP wrapper class is generated which calls a set of flat functions wrapping the C++ class.

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.

#### 29.2.6.1 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);

#### 29.2.6.2 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)`

#### 29.2.6.3 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 has 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 has now been " . Ko::threats() . " threats\n";

#### 29.2.6.4 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();

### 29.2.7 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 `%init` or `%minit`.

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

The `%rinit` and `%rshutdown` statements insert code into the request init and shutdown code respectively.