interface.php

<?php

interface iBase {
  public function getData();
}

class SampleA implements iBase {
	public function getData() {
		return 'A';
	}
}

class SampleB implements iBase {
	public function getData() {
		return 'B';
	}
}

function echoObj(iBase $obj) {
	echo $obj->getData();
}

echoObj(new SampleA());
echoObj(new SampleB());