In Part 1 we understood that __autoload() works well when all our classes are at one place and follows same naming conventions. However in real world application, classes are divided in different modules and packages, placed at multiple locations. This situation can be handled using __autoload by checking multiple locations inside the __autoload as shown in the following example

function __autoload($className) {
    $path1 = '/var/www/prj/user/';
    $path2 = '/var/www/prj/email/';
    if(file_exists($path1.$className.'.php')) {
        require_once($path1.$className.'.php');
    } else if(file_exists($path2.$className.'.php')) {
        require_once($path2.$className.'.php');
    }
}

but still __autoload can only be used once in the entire application and with the increasing use of third party libraries and large code base __autoload will become bulkier and hard to maintain.

spl_autoload_registor() solves the problem of using large number of third-party library classes. it gives us option to create multiple functions like __autoload to be called one by one until class is loaded or all registered functions get called. Following example explain the basic use of spl_autoload_registor()

File : /var/www/html/autoloading/recommended/User.php

class User {
	public function __construct() {
		echo 'User class found'.PHP_EOL;
	}
}

File : /var/www/html/autoloading/recommended/load.php

function loadClasses($argument) {

	require_once $argument.'.php';

	echo 'autoload get called'.PHP_EOL;
}

spl_autoload_register('loadClasses');

File : /var/www/html/autoloading/recommended/recommended-example.php

require_once 'load.php';

$user = new User();

Output:
autoload get called
User class found

in the above example loadClasses is registered to load our classes instead of __autoload().

Note: __autoload will not get called if spl_autoload_register is used.

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Autoload is the programming concept to load and link the portion of a program from mass storage automatically when needed. With increase of classes in PHP projects, PHP introduce auto-loading for loading classes automatically.

Lets understand the basics of autoloading step by step.

Create a class

File : /var/www/html/autoloading/basic/User.php

class User {
	public function __construct() {
		echo 'User class found'.PHP_EOL;
	}
}

Lets try to use the class

File : /var/www/html/autoloading/basic/basic-example.php

require_once 'User.php';

$user = new User();

Output:
User class found.

But wait, this is the old way of including class definitions, what if we have 100 classes in the project, do we need to put 100 includes, yes we need to but then why auto loading feature is introduced. Lets try to use the auto loading.

File : /var/www/html/autoloading/basic/basic-example.php [ updated to use autoloading ]

// require_once 'User.php';

$user = new User();

Output:
PHP Fatal error: Class ‘User’ not found in /var/www/html/autoloading/basic/basic-example.php on line 4

Confused why this output, PHP has autoloading feature, class should be found. Yes output is correct automatically does not mean PHP found all your classes by some magic and you do not have to do anything. In auto-loading PHP gives a last chance to load a class definition if not found by calling a function ‘__autoload()’. It is the developers responsibility to implement __autoload().

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