Little mistake on naming variable make me confuse for almost an hour.
$this
yes, $this is built-in variable that often use in OOP. By using $this, we refer to self referencing variable.
More in php.net manual

<?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();
}
}
$a = new A();
$a->foo();
A::foo();
$b = new B();
$b->bar();
B::bar();
?>

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