$this

php.net

<?php


class A 
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (',get_class($this),')'."\n";
        } else {
            echo '$this is not defined.'."\n";
        }    
    }
}

class B 
{
    function bar()
    {
        A::foo();
    }
}

$a = new A();
$a->foo();

A::foo();
$b = new B();
$b->bar();

B::bar();

$this is defined (A) 

( ! ) Strict standards: Non-static method A::foo() should not be called statically in D:\wamp64\www\w0827pm\w.php on line 27
Call Stack
#	Time	Memory	Function	Location
1	0.0020	241448	{main}( )	...\w.php:0

$this is not defined. 

( ! ) Deprecated: Non-static method A::foo() should not be called statically, assuming $this from incompatible context in D:\wamp64\www\w0827pm\w.php on line 20
Call Stack
#	Time	Memory	Function	Location
1	0.0020	241448	{main}( )	...\w.php:0
2	0.0030	242352	B->bar( )	...\w.php:29

$this is defined (B) 

( ! ) Strict standards: Non-static method B::bar() should not be called statically in D:\wamp64\www\w0827pm\w.php on line 31
Call Stack
#	Time	Memory	Function	Location
1	0.0020	241448	{main}( )	...\w.php:0

( ! ) Strict standards: Non-static method A::foo() should not be called statically in D:\wamp64\www\w0827pm\w.php on line 20
Call Stack
#	Time	Memory	Function	Location
1	0.0020	241448	{main}( )	...\w.php:0
2	0.0030	242352	B::bar( )	...\w.php:31

$this is not defined.

/*

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.

*/