How to call __autoload() in php code

I am new to php and I want to use php5's __autoload functionality in my code. I wrote below code in my index.php but I don't understand how and when I should call __autoload function.

function __autoload($class) {
    if (file_exists($class . '.php')) {
        include($class . '.php');
    } else {
        throw new Exception('Unable to load class named $class');
    }
}

I have seen this thread also but I do not have such autoloader class in my application. Do every application need a separate class in order to use autoloading? If not can I have a single function like above and complete it's task? Can anyone explain how to call above __autoload function inside my php code?

You don't call __autoload() yourself, PHP does when it is trying to find the implementation of a class.

For example...

function __autoload($className) {
     include $className . '.php';
}

$customClass = new CustomClass;

...this will call __autoload(), passing CustomClass as an argument. This (stupid for example's sake) __autoload() implementation will then attempt to include a file by tacking the php extension on the end.

As an aside, you should use spl_autoload_register() instead. You can then have multiple implementations, useful when using multiple libraries with auto loaders.

...using __autoload() is discouraged and may be deprecated or removed in the future.

Source.