$this¶

$this is a variable that represents the current object. It is different in each object, each method.

$this is also called a pseudo-variable. It is never explicitly set, but is available as soon as the method starts.

$this cannot be unset, nor assigned a different value than the current object. This means it cannot be used as an argument, for example.

There is no special variable called $that.

<?php

    class X {
        private $y = 1;

        function foo() {
            return $this->y;
        }
    }

?>

Related : Class, Trait