get_defined_vars is inconsistent with $this

<?php

class Test
{
    public $var = 4;

    public function doStuff() {
        $this->var = 4;
    }
    public function testNoThis() {
        echo __METHOD__ . PHP_EOL;
        var_dump(array_keys(get_defined_vars()));
    }
    public function testThisGet() {
        echo __METHOD__ . PHP_EOL;
        var_dump(array_keys(get_defined_vars()));
        $this->var;
    }
    public function testThisSet() {
        echo __METHOD__ . PHP_EOL;
        var_dump(array_keys(get_defined_vars()));
        $this->var = 4;
    }

    public function testThisCall() {
        echo __METHOD__ . PHP_EOL;
        var_dump(array_keys(get_defined_vars()));
        $this->doStuff();
    }

    public function testThisUse() {
        echo __METHOD__ . PHP_EOL;
        var_dump(array_keys(get_defined_vars()));
        $foo = $this;
    }
}

$t = new Test();
$t->testNoThis();
$t->testThisGet();
$t->testThisSet();
$t->testThisCall();
$t->testThisUse();