spl_autoload()

(PHP 5 >= 5.1.0, PHP 7)

__autoload()函数的默认实现

说明

本函数提供了__autoload()的一个默认实现。如果不使用任何参数调用 spl_autoload_register()函数，则以后在进行 __autoload()调用时会自动使用此函数。

参数

在默认情况下，本函数先将类名转换成小写，再在小写的类名后加上.inc 或.php 的扩展名作为文件名，然后在所有的包含路径(include paths)中检查是否存在该文件。

返回值

没有返回值。

Note, that the default autoload implementation is written in C land and is always slightly faster then your native PHP one.
Here is a trick to use default implementation with any configuration:

This also works with namespaces out of the box. So you can write code like "use My\Name\Object" and it will map to "class/My/Name/Object.class.php" file path!

If you want to make the best use out of autoload with an APC cache don't use spl_autoload. It uses relative paths and thus will perform a stat even with apc.stat=0 (either that, or it doesn't work at all).
Instead make a custom function and use require/include with an absolute path (register it with spl_autoload_register).
Do NOT use *_once functions or a relative path. This will fail harder than spl_autoload.
Also avoid using file_exists and is_file. This will also perform a stat.
Why are stats bad? Because they access the file system. PHP does have a stat cache that helps, but it defeats the purpose of apc.stat = 0.
It's also good to keep in mind that it's good to keep your custom autoload function simple. This is my Loader class:

ClassB.php

ClassC.php

ClassD.php

ClassE.php

1. Simple:

2. Change priority:

Or you can use this simple function that runs a bit faster for the extensions with lower priority :)

---
Safak Ozpinar - Istanbul University, Computer Engineering

Note this function will LOWERCASE the class names its looking for, dont be confused when it cant find Foo_Bar.php
also, unlike most other autoloader code snippets, this function DOES NOT translate underscores to slashes.
class Foo_Bar {}
will load foo_bar.php and will not try to load foo/bar.php
You can get around this with
spl_autoload_register(function($class) { return spl_autoload(str_replace('_', '/', $class));});

One small example that shows how you can use spl_autoload function in your MVC, Framewrk's applications. For example, will use the Loader class. 

Just thought I'd react to simast at gmail dot com's note: While he has a point in saying C outperforms PHP, his suggestion is micro-optimization. I'm not 100% against micro-optimizing code, but if you do, go all the way:

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