return true

1.php

<?php

function foo($x)
{
    return $x === $x();
}

2.php

<?php

class Bar {}

function foo(Bar $x)
{
    return get_class($x) != 'Bar';
}

3.php

<?php

class Bar
{
    private $a;
    public function __construct($a)
    {
        $this->a = (bool) $a;
    }
    public function a()
    {
        return $this->a;
    }
}

function foo(callable $x)
{
    $object = new Bar(false);
    $x($object);
    return $object->a();
}

4.php

<?php

class Test
{
    private $foo = true;
}

function foo($x)
{
    $o = (array) new Test;
    return $o[$x];
}

4

✅ foo("\0Test\0foo")

read more how it works - http://ocramius.github.io/blog/fast-php-object-to-array-conversion

3

✅ Closure::bind(function(Bar $obj) { $obj->a = true; }, null, 'Bar')

read more how it works - https://ocramius.github.io/blog/accessing-private-php-class-members-without-reflection/

2

✅ foo(new class() extends Bar {})

1

✅ foo($a = function() use (&$a) { return $a; })