<?php
/*class ReplacementClass {
	public function foo() {
		return "foo";
	}
	public function bar() {
		return "bar";
	}
}*/
if(!class_exists('ReplacementClass')) {
   class MyReplacementClass {
     public function foo() {
     	echo " foo does not exist, this is a placeholder";
     }
     public function bar() {
     	echo " bar does not exist, this is a placeholder";
     }
   }
}
else {
   class MyReplacementClass extends ReplacementClass {
   }
}

class FooBar extends MyReplacementClass {

}

$x = new FooBar();
echo $x->foo() . $x->bar() . "\n";