About loading classes in PHP

Object-oriented is an important idea, and classes are also important concepts in object-oriented, but class loading is the key to using classes.

There are two ways to access classes:

• Access through instantiated objects

• Access to class members

The prerequisite for access is that there is a class in the memory, so the class needs to be loaded into the memory in advance.

1. Manual loading

//类文件 Salary.php
<?php
   class Salary{
      public  function Student(){
        echo "Salary下面的Student方法";
      }
   }
?>

应用文件：useSalary.php
<?php
//$s = new Salary();会报错，因为useSalaty.php中间未定义Salary类

include_once &#39;Salary.php&#39;;              //也可以使用require，通常使用_once，因为类不允许重名
$s = new Salary();
echo $s->Student();
?>

<?php
//加载类文件是一种比较消耗资源的方式,可以事先使用class_exists()函数来判定类是否存在，存在就不用加载，不存在才加载
if(!class_exists(&#39;Salary&#39;)){
    //不存在：加载
    include_once &#39;Salary.php&#39;;
}
//使用
$s = new Salary();
?>

2. Automatic loading

• The automatic loading mechanism used before PHP7: Use the __autoload() function provided by the system. Then when the system needs to use the class and it does not exist in the memory, the system will automatically call the __autoload() function to load the class file. .

<?php
function __autoload($classname){    //参数为类名：即当前需要访问的类的名字
//需要人为定义去哪加载，怎么加载
    include_once $classname . &#39;.php&#39;;   //假定为当前目录下，类文件名字为：类名.php
}

//使用类：内存目前并没有
$s = new Salary();  //系统发现内存没有Salary，所以调用__autoload()去加载
?>

//若在不同路径下
<?php
//定义自动加载
function __autoload($classname){
    $abc_file = &#39;abc/&#39; . $classname . &#39;.php&#39;;       //如abc/Salary.php
    if(file_exists($c_file)){                      //利用file_exists()判断文件是否存在
        include_once $abc_file;
    }
}
?>

• After PHP7, it is not recommended to use the __autoload() function, but to use a registration mechanism to customize the user The function is placed inside the system and uses spl_autoload_register (defined function).

<?php
function myself_autoload($classname){       //与__autoload()类似
    $abc_file = &#39;abc/&#39; . $classname . &#39;.php&#39;;       //如abc/Salary.php
    if(file_exists($c_file)){
        include_once $c_file;
    }
}
//此时，上述函数永远不会自动运行，除非将函数注册到系统内部
spl_autoload_register(&#39;myself_autoload&#39;);
?>

//可以定义多个方法
<?php
function wayone_autoload($classname){

function waytwo_autoload($classname){

}
//此时，上述函数永远不会自动运行，除非将函数注册到系统内部
spl_autoload_register(&#39;wayone_autoload&#39;);
spl_autoload_register(&#39;waytwo_autoload&#39;);
?>

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