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+<div style="width: 1024px; font-family: verdana; font-size: 10pt; line-height: 16pt;">
+
+
+<h1>How does PHP load its extensions?</h1>
+<p>
+    You probably already know that native PHP extensions are compiled into *.so 
+    files on unix-like systems, and *.dll files in Windows environments, and that
+    the global php.ini file holds a list of all extensions available on your system.
+    This means that if you're building your own extension, you are also going to
+    create such a *.so or *.dll file and you have to update the PHP 
+    configuration file so that your own extension is loaded by PHP.
+</p>
+<h3>Where to find your PHP configuration files?</h3>
+<p>
+    If for one reason or another you can not find the PHP configuration file(s) 
+    on your system, you can run the following command from the command line:
+</p>
+<p>
+    <code><pre>
+        php --ini
+    </pre></code>
+</p>
+<p>
+    This will output a list of all configuration files that are loaded by PHP.
+    Extensions are enabled by adding "extension=name.so" lines to the
+    configuration file - where 'name' should of course be replaced by the name of 
+    your extension. A default PHP installation already comes with many default 
+    extensions, so in the configuration file(s) on your system you will certainly 
+    find a number of these "extension=name.so" lines.
+</p>
+<p>
+    The extension lines either take an absolute path ("extension=/path/to/extension.so")
+    or a relative path ("extension=extension.so"). If you'd like to use relative
+    paths, you must make sure that you've copied your extension *.so file to the
+    default extension directory, so that PHP can find it. To find out this default
+    extension directory, use the following command line instruction:
+</p>
+<p>
+    <code><pre>
+        php -i|grep extension_dir
+    </pre></code>
+</p>
+<p>
+    The extension dir often has the form /usr/lib/php5/20121212 - or a different
+    date string depending on the PHP version you use.
+</p>
+
+<h2>The get_module() startup function</h2>
+<p>
+    Before we explain how you can create your own extension, we first explain
+    what PHP does to load an extension. When PHP starts, it loads the *.ini configuration 
+    file(s) that we just described and for each "extension=name.so" line in these
+    files, it opens the appropriate library, and calls the "get_module()"
+    function from it. Each extension library (your extension too) must therefore 
+    define and implement this "get_module()" C function. The function is called by 
+    PHP right after the library is loaded (and thus way before pageviews are handled), 
+    and it should return a memory address that points to a structure that holds information 
+    about all functions, classes, variables and constants that are made available 
+    by the extension. 
+</p>
+<p>
+    The structure that the get_module() returns is defined in the header files of 
+    the Zend engine, but it is a pretty complicated structure without good documentation. 
+    Luckily, the PHP-CPP library makes life easier for you, and offers an Extension 
+    class that can be used instead.
+</p>
+<p>
+    <code><pre>
+        #include &lt;phpcpp.h&gt;
+        
+        /**
+         *  tell the compiler that the get_module is a pure C function
+         */
+        extern "C" {
+            
+            /**
+             *  Function that is called by PHP right after the PHP process
+             *  has started, and that returns an address of an internal PHP
+             *  strucure with all the details and features of your extension
+             *
+             *  @return void*   a pointer to an address that is understood by PHP
+             */
+            PHPCPP_EXPORT void *get_module() 
+            {
+                // static(!) Php::Extension object that should stay in memory
+                // for the entire duration of the process (that's why it's static)
+                static Php::Extension myExtension("my_extension", "1.0");
+                
+                // @todo    add your own functions, classes, namespaces to the extension
+                
+                // return the extension
+                return myExtension;
+            }
+        }
+    </pre></code>
+</p>
+<p>
+    In the example above you see a very straightforward implementation of the
+    get_module() function. Every PHP extension that uses the PHP-CPP library
+    implements this function in a more or less similar way, and it is the 
+    starting point of each extension. A number of elements require special attention. 
+    For a start, the only header file that you see is the phpcpp.h header
+    file. If you're using the PHP-CPP library to build your own extensions,
+    you do not have to include the complicated, unstructured, and mostly undocumented
+    header files of the Zend engine - all you need is this single phpcpp.h header 
+    file of the PHP-CPP library. If you insist, you are of course free to also 
+    include the header files of the core PHP engine - but you do not have to. 
+    PHP-CPP takes care of dealing with the internals of the PHP engine, and offers 
+    you a simple to use API.
+</p>
+<p>
+    The next thing that you'll notice it that we placed the get_module() function
+    inside an 'extern "C"' code block. As the name of the library already gives away, 
+    PHP-CPP is a C++ library. However, PHP expects your library, and especially your 
+    get_module() function, to be implemented in C and not in C++. That's why we've 
+    wrapped the get_module() function in an 'extern "C"' block. This will instruct 
+    the C++ compiler that the get_module() is a regular C function, and that it
+    should not apply any C++ name mangling to it.
+</p>
+<p>
+    The PHP-CPP library defines a "PHPCPP_EXPORT" macro that should be placed
+    in front of the get_module() function. This macro makes sure that the get_module()
+    function is publicly exported, and thus callable by PHP. The macro has a different
+    implementation based on the compiler and operating system.
+</p>
+<p>
+    This, by the way, is also the only macro that PHP-CPP offers. PHP-CPP intends to 
+    be a very straightforward C++ library, without using magic or tricks from 
+    pre-processors. What you see is what you get: If something looks like a 
+    function, you can be sure that it actually IS a function, and when something 
+    looks like a variable, you can be sure that it also IS a variable.
+</p>
+<p>
+    Let's move on. Inside the get_module() function the Php::Extension object is 
+    instantiated, and it is returned. It is crucial that you make a <i>static</i>
+    instance of this Php::Extension class, because the object must exist for the 
+    entire lifetime of the PHP process, and not only for the duration of the get_module()
+    call. The constructor takes two arguments: the name of your extension and 
+    its version number.
+</p>
+<p>
+    The final step in the get_module() function is that the extension object
+    is returned. This may seem strange at first, because the get_module() function
+    is supposed to return a pointer-to-void, and not a full Php::Extension object.
+    Why does the compiler not complain about this? Well, the Php::Extension class 
+    has a cast-to-void-pointer-operator. So although it seems that you're returning 
+    the full extension object, in reality you only return a memory address that
+    points to a data structure that is understood by the core PHP engine and that
+    holds all the details of your extension.
+</p>
+<p>
+    Note that the example above does not yet export any native functions or 
+    native classes to PHP - it only creates the extension. That is going to be 
+    the next step.
+</p>
+
+
+
+<h2>Exporting native functions</h2>
+<p>
+    An extension can of course only be useful if you define functions and/or 
+    classes that can be accessed from PHP scripts. For functions you can do this 
+    by adding your native function implementations to the Extension object:
+</p>
+<p>
+    <code><pre>
+        #include &lt;phpcpp.h&gt;
+        
+        extern void example1();
+        extern void example2(Php::Parameters &amp;params);
+        extern Php::Value example3();
+        extern Php::Value example4(Php::Parameters &amp;params);
+        
+        extern "C" {
+            PHPCPP_EXPORT void *get_module() {
+                static Php::Extension myExtension("my_extension", "1.0");
+                myExtension.add("native1", example1);
+                myExtension.add("native2", example2);
+                myExtension.add("native3", example3);
+                myExtension.add("native4", example4);
+                return myExtension.module();
+            }
+        }
+    </pre></code>
+</p>
+<p>
+    What do we see here? We've added four function declarations ("example1", 
+    "example2", "example3" and "example4") to the source code of our extension. 
+    The reason why we've only declared the functions, and not fully implemented 
+    them is to keep the example code relatively small. We assume that the 
+    four example functions are implemented in a different file. In a real world 
+    example you could just as well remove the "extern" keyword and implement the 
+    four functions in the same source file as the get_module() call.
+</p>
+<p>
+    The four functions all have a different signature: Some return a value, while
+    others do not return anything. And some take parameters, while others do not.
+    Despite the different signature of the functions, they can all be made
+    available in PHP by adding them to the extension object, by simply calling
+    the myExtension.add() method. This method takes two parameters: the name by
+    which the function should be accessible in PHP, and the actual native
+    function.
+</p>
+<p>
+    In the example above we've used different names for the native functions
+    ("example1" up to "example4") as for the PHP functions ("native1" to
+    "native4"). This is legal - you do not have to use the same names for your
+    native functions as for your PHP functions. The following PHP script can be 
+    used to call the four native functions:
+</p>
+<p>
+    <code><pre>
+        &lt;?php
+        native1();
+        native2("a","b");
+        $x = native3();
+        $y = native4(1,2);
+        ?&gt;
+    </pre></code>
+</p>
+<p>
+    It is not possible to export every thinkable C/C++ function to the 
+    PHP extension. Only functions that have one of the four supported signatures
+    can be exported: functions that return
+    void or a Php::Value object, and that either accept a Php::Parameters object
+    or no parameters at all, can be added to the extension object and can thus
+    be exported to PHP.
+</p>
+<h2>Parameter types</h2>
+<p>
+    PHP has a mechanism to enforce function parameters types, and to accept
+    parameters either by reference or by value. In the examples above, we have
+    not yet used that mechanism yes: it is up to the function implementations
+    themselves to inspect the 'Parameters' object, and check if the
+    variables are of the right type.
+</p>
+<p>
+    However, the 'Extension::add()' method takes a third optional parameter that
+    you can use to specify the number of parameters that are supported, whether
+    the parameters are passed by reference or by value, and what the type of
+    the parameters is:
+</p>
+<p>
+    <code><pre>
+        #include &lt;phpcpp.h&gt;
+        
+        extern void example(Php::Parameters &amp;params);
+        
+        extern "C" {
+            PHPCPP_EXPORT void *get_module() {
+                static Php::Extension myExtension("my_extension", "1.0");
+                myExtension.add("example", example, {
+                    Php::ByVal("a", Php::Type::Numeric),
+                    Php::ByVal("b", "ExampleClass"),
+                    Php::ByRef("c", "OtherClass")
+                });
+                return myExtension.module();
+            }
+        }
+    </pre></code>
+</p>
+<p>
+    Above you see that we passed in additional information when we registered the
+    "example" function. We tell our extension that our function accepts three parameters:
+    the first parameter must be a regular number, while the other ones are object 
+    instances of type "ExampleClass" and "OtherClass". In the end, your native C++ 
+    "example" function will still be called with a Php::Parameters instance, but 
+    the moment it gets called, you can be sure that the Php::Parameters object 
+    will be filled with three members, and that two of them are objects of the 
+    appropriate type, and that the third one is also passed by reference.
+</p>
+<h2>Working with variables</h2>
+<p>
+    Variables in PHP are non-typed. A variable can thus hold any possible type:
+    an integer, string, a floating point number, and even an object or an array.
+    C++ on the other hand is a typed language. In C++ an integer variable always 
+    has a numeric value, and a string variable always hold a string value.
+</p>
+<p>
+    When you mix native code and PHP code, you will need to convert the non-typed 
+    PHP variables into native variables, and the other way round: convert native 
+    variables back into non-typed PHP variables. The PHP-CPP library offers the 
+    "Value" class that makes this a very simple task.
+</p>
+<p>
+    
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