PHP 面向对象语法细节

1. $this伪变量

   The pseudo-variable $this is available when a method is called from within an object context. $this is a reference to the calling object (usually the object to which the method belongs, but possibly another object, if the method is called statically from the context of a secondary object).
   As of PHP 7.0.0 calling a non-static method statically from an incompatible context results in $this being undefined inside the method. Calling a non-static method statically from an incompatible context has been deprecated as of PHP 5.6.0.
   As of PHP 7.0.0 calling a non-static method statically has been generally deprecated (even if called from a compatible context). Before PHP 5.6.0 such calls already triggered a strict notice.

   即

   1. 伪变量$this在object上下文中方法被调用时有效，它是"the calling object"的引用，（通常，the calling object 是方法所属的对象。但是，也可能是其他对象，如果方法是从次级对象（我的理解是子类对象）的上下文中静态调用）。
   2. 如果静态地调用一个非静态的方法，
             1. 在PHP 5.6.0之前，不论from an incompatible context 或者 from compatible context 静态调用非静态方法都会触发a strict notice，从incompatible context静态调用非静态方法，$this对象是secondary object的引用，如下面例子中的31行，最后输出指明$this指向B类的对象。
             2. 在PHP 5.6.0之后，from an incompatible context静态调用一个非静态方法会引发deprecated。其他情况静态调用非静态方法会触发a strict notice。并且from an incompatible context 静态调用一个非静态方法，非静态方法中的$this伪变量保存secondary object的引用。
             3. 在PHP 7.0.0之后，不论是从an incompatible context 或者 from compatibel context 静态调用非静态方法，都会引发deprecated。在PHP7.0.0中，from an incompatible context 静态调用非静态方法会导致非静态方法内的$this伪对象是undefined，与PHP 7.0.0之前，$this伪对象是secondary object 的引用不同。

   例子：
   

   <?php
   class A
   {
       function foo()
       {
           if (isset($this)) {
               echo '$this is defined (';
               echo get_class($this);
               echo ")\n";
           } else {
               echo "\$this is not defined.\n";
           }
       }
   }
   
   class B
   {
       function bar()
       {
           A::foo();
       }
   }
   
   error_reporting(E_ALL);
   
   $a = new A();
   $a->foo();                 //从compatible context调用方法
   
   A::foo();　　              //从compatible context 静态调用非静态方法
   $b = new B();
   $b->bar();                //从一个incompatibale context静态调用非静态方法
   
   B::bar();                 //从compatible context 静态调用非静态方法bar(),然后在bar() 从compatible context中再次静态调用非静态方法
   ?>

   php 5.5.38执行效果
   
   
   php 5.6.30执行效果
   
   
   php 7.1.2执行效果