How to override method for one object in PHP?

Question

I need is to override a method for a single instance

I just can't override any method with the algorithms I took here: PHP override function of a single instance

My code:

class Test {
    public function testFunc() {
        echo "1";
    }
}

$a = new Test();

# I tried another solution too and it doesn't work
$a->testFunc = Closure::bind(function() {
    echo "2";
}, $a);

$a->testFunc();

After calling testFunc() I see 1, not 2.

Where is the mistake?