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PHP: Callback functions and OOP

Recently, I had to change default behavior of storing session data into files and use MySQL DB instead. In practice, that means writing whole bunch of callback functions and setting callbacks with session_set_save_handler function. Since I use OOP, what really bothered me was the fact that (according to my PHP CHM Manual sitting on my desktop) for session_set_save_handler, all functions has to exist in global scope, since all callback arguments are strings?

bool session_set_save_handler ( string open, string close, string read, string write, string destroy, string gc )

Doing that in non OOP way with 6 functions on global scope is not something I really liked, so I googled for solution and found that you can easily assign an array like array(‘class_name’, ‘method’) for all callbacks in PHP. Cool stuff which allows you to create session handler class with bunch of static methods for those callbacks, but why the hell that is not documented in PHP Manual???

I went to online manual at least to see if someone submitted comment about this, and find out that session_set_save_handler definition there is completely different:

bool session_set_save_handler ( callback $open, callback $close, callback $read, callback $write, callback $destroy, callback $gc )

Obviously, since last time I browsed online manual, a lot of thing has changed, one among them is introducing “callback” type in those “pseudo types” used only for documentation purposes. And there, manual for callback says following:

callback

Some functions like call_user_func() or usort() accept user defined callback functions as a parameter. Callback functions can not only be simple functions but also object methods including static class methods.

A method of an instantiated object is passed as an array containing an object as the element with index 0 and a method name as the element with index 1.

Static class methods can also be passed without instantiating an object of that class by passing the class name instead of an object as the element with index 0.

which basically allows you to pass an array with class name and method as callback, and that method will be called.

Let me give you and example with sessions:

<?php

/**
 * Sessin_Handlers class
 * contains dummy methods needed for session stuff
 * Replace content with some real stuff like db conn etc.
 *
 */
class Session_Handlers
{
	function open($save_path, $session_name)
	{
		echo "Open Method Called<br>";
		return true;
	}

	function close()
	{
		echo "Close Method Called<br>";
		return true;
	}

	function read($id)
	{
		echo "Read Method Called<br>";
		return true;
	}

	function write($id, $sess_data)
	{
		echo "Write Method Called<br>";
		return true;

	}

	function destroy($id)
	{
		echo "Destroy Method Called<br>";
		return true;
	}

	function gc($maxlifetime)
	{
		echo "GC Method Called<br>";
		return true;
	}
}

//call all method from Session_Handlers statically
session_set_save_handler(array('Session_Handlers', 'open'), array('Session_Handlers', 'close'), array('Session_Handlers', 'read'), array('Session_Handlers', 'write'), array('Session_Handlers', 'destroy'), array('Session_Handlers', 'gc'));

session_start();

// proceed to use sessions normally
?>

As you see, we’ve created simple methods which only echo when they are called (in real life, you should either save session data into file or db). As you can see, we simple passed arrays to session_set_save_handler, which served us to connect class methods with session callbacks.

7 thoughts to “PHP: Callback functions and OOP”

  1. Exactly what I needed, thank you.
    Also, for completeness, here is an example of calling a method of the current class, when a static method won’t work:
    class My_Class {
    var $id_lst = array();
    function procLinks() {

    $links = array_map(array($this, addLinkId), $links);

    }
    function addLinkId($link) {

    $this->id_lst[$id] = 1;

    }

  2. Great stuff, was struggling with this for about 20mins until I gave up and searched the web. Thanks for the info!

  3. Good stuff, works with preg_replace_callback as well. I also found out if you feed the callback array($this, “my_function”) it will use the current instance of the class. I haven’t tried yet but I would assume you could use any instance of a class with that function.

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