PHP 面向对象语法细节

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，

例子：

 1 <?php
 2 class A
 3 {
 4     function foo()
 5     {
 6         if (isset($this)) {
 7             echo ‘$this is defined (‘;
 8             echo get_class($this);
 9             echo ")\n";
10         } else {
11             echo "\$this is not defined.\n";
12         }
13     }
14 }
15
16 class B
17 {
18     function bar()
19     {
20         A::foo();
21     }
22 }
23
24 error_reporting(E_ALL);
25
26 $a = new A();
27 $a->foo();
28
29 A::foo();　　              //静态调用非静态方法
30 $b = new B();
31 $b->bar();                //从一个incompatibale context静态调用非静态方法
32
33 B::bar();                 //静态调用非静态方法bar(),然后在bar()中再次静态调用非静态方法
34 ?>

php 5.5.38执行效果

php 7.1.2执行效果