如果我尝试从其他函数内部创建类的实例，则自动加载不起作用

helper.php : Required at the top of index.php

function __autoload($className) {

    $path = ROOT_DIR.'/class/'.strtolower(trim($className)).'.class.php';

    if (file_exists($path)) {
        require_once($path);
    }
}

function load_libs($class_variable_name) {

    global $session;
    global $_libs;

    $_libs[$class_variable_name] = new $class_variable_name($session);

    return $_libs[$class_variable_name];
}

From index.php I create an instance of a class :

new some_class_one();

No problem, it loads class file via __autoload.

But, in case that I call :

load_libs('some_class_two');

I getting error : Fatal error: Class 'some_class_two' not found in C:\wamp\www\helper.php on line

Any idea what to do ?

I just tested this on my machine and it works perfectly. I could not replicate the issue. This is the version of PHP I'm running:

$ php -v
PHP 5.4.17 (cli) (built: Aug 25 2013 02:03:38) 
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies

I would suggest that you remove the .class from your filenames. It's better to use namespaces and have the namespace/class name match the filesystem. Other than that, either your ROOT_DIR is incorrect or your file is located in the wrong place.