php automatic loading method

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php loading File method:
1, include, include_once, requice, requice_one regular loading
2, __autoload()

3. spl_autoload_register()

##Conventional loading method Assume we have a class file A.php, It defines a class named A:

 Then we have an index.php that needs to use this class A. The conventional writing method is 
 

But there is a problem, if our index.php needs to contain not only class A, but many classes, in this way, many lines of require must be written Statements can sometimes make people feel uncomfortable. 

__autoload() automatically loads 
But in versions after php5, we no longer need to do this. In php5, the __autoload function is automatically called when trying to use a class that has not been defined, so we can write the __autoload function to let php automatically load the class without having to write a long list of include files. 
For example, in the above example, index.php can be written like this: 


Of course the above is just the simplest demonstration, __autoload just goes include_path finds class files and loads them. We can define __autoload rules for loading classes according to our own needs. Note: Since __autoload() is a function, it can only exist once. 

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：

或者直接使用匿名函数：

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：

更多示例
autoload.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。
以上就是本文的全部内容，希望对大家的学习有所帮助。
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