Summary of automatic loading methods in php
这篇文章主要针对php自动加载方式进行大集合，一种是常规加载方式，另一种是__autoload()自动加载，想知道第三种方式请阅读下文
php加载文件方式：
1、include,include_once,requice,requice_one常规加载
2、__autoload()
3、spl_autoload_register()
常规加载方式
假设我们有一个类文件A.php，里面定义了一个名字为A的类：
<?php
class A {
public function __construct() {
echo 'Got it.';
}
}然后我们有一个index.php需要用到这个类A，常规的写法就是
<?php require('A.php'); $a = new A();
但是有一个问题就是，假如我们的index.php需要包含的不只是类A，而是需要很多类，这样子就必须写很多行require语句，有时候也会让人觉得不爽。
__autoload()自动加载
不过在php5之后的版本，我们就不再需要这样做了。
在php5中，试图使用尚未定义的类时会自动调用__autoload函数，所以我们可以通过编写__autoload函数来让php自动加载类，而不必写一个长长的包含文件列表。
例如在上面那个例子中，index.php可以这样写：
<?php
function __autoload($class){
$file = $class . '.php';
if (is_file($file)) {
require_once($file);
}
}
$a = new A();当然上面只是最简单的示范，__autoload只是去include_path寻找类文件并加载，我们可以根据自己的需要定义__autoload加载类的规则。注意：由于__autoload()是个函数，只能存在一次。
spl_autoload_register()自动加载
但现在问题来了，如果在一个系统的实现中，如果需要使用很多其它的类库，这些类库可能是由不同的开发人员编写的，其类名与实际的磁盘文件的映射规则不尽相同。这时如果要实现类库文件的自动加载，就必须在__autoload()函数中将所有的映射规则全部实现，这样的话__autoload()函数有可能 会非常复杂，甚至无法实现。最后可能会导致__autoload()函数十分臃肿，这时即便能够实现，也会给将来的维护和系统效率带来很大的负面影响。在这种情况下，难道就没有更简单清晰的解决办法了吧？答案当然是：NO!
spl_autoload_register() 满足了此类需求。 它实际上创建了 autoload 函数的队列，按定义时的顺序逐个执行。相比之下， __autoload() 只可以定义一次。
bool spl_autoload_register ([ callable $autoload_function [, bool $throw = true [, bool $prepend = false ]]] )
我们继续改写上面那个例子：
<?php
function loader($class){
$file = $class . '.php';
if (is_file($file)) {
require_once($file);
}
}
spl_autoload_register('loader');
$a = new A();或者直接使用匿名函数：
<?php
spl_autoload_register(function($file){
$file = $class . '.php';
if (is_file($file)) {
require_once($file);
}
});
$a = new A();这样子也是可以正常运行的，这时候php在寻找类的时候就没有调用__autoload而是调用我们自己定义的函数loader了。同样的道理，下面这种写法也是可以的：
<?php
class Loader {
public static function loadClass($class){
$file = $class . '.php';
if (is_file($file)) {
require_once($file);
}
}
}
spl_autoload_register(array('Loader', 'loadClass'));
//spl_autoload_register(array(__CLASS__, 'loadClass'));
//spl_autoload_register(array($this, 'loadClass'));
$a = new A();更多示例
autoload.php
<?php
define('SDK_PATH', __DIR__);
require_once SDK_PATH . '/common/functions.php';
require_once SDK_PATH . '/common/config.php';
spl_autoload_register(function ($class) {
if (false !== stripos($class, 'YJC\Wechat')) {
require_once __DIR__ . '/' . str_replace('\\', DIRECTORY_SEPARATOR, substr($class, 10)) . '.php';
}
});
/*
function __autoload($class){
if (false !== stripos($class, 'YJC\Wechat')) {
require_once __DIR__ . '/' . str_replace('\\', DIRECTORY_SEPARATOR, substr($class, 10)) . '.php';
}
}
*/建议入口文件里养成定义绝对地址SDK_PATH的习惯，这样require不会出错。
总结：以上就是本篇文的全部内容，希望能对大家的学习有所帮助。
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