Generic PHP interface (version < 5.4.1)

test.php

<?php
/// Work in PHP < 5.4.1

interface ITest {
    public function testSelf(self $test);
}

class Test implements ITest {
    private $a = 10;
    
    public function testSelf(self $test) {
        echo $test->a;
    }
}

class Test2 extends Test {
    private $a = 11;
    
    public function testSelf(self $test) {
        echo $test->a;
    }
}
$test = new Test();
$test->testSelf($test);
echo PHP_EOL;
$test2 = new Test2();
$test2->testSelf($test2);

// result:
// 10
// 11