test code

 <?php
     abstract class Mediator{
         abstract public function send($message, $colleague);
     }

     abstract class Colleague{
         private $_mediator = null;
         public function __construct($mediator){
             $this->_mediator = $mediator;
         }

         public function send($message){
             $this->_mediator->send($message, $this);
         }

         abstract public function notify($message);
     }

     class ConcreteMediator extends Mediator{
         private $_colleague1 = null;
         private $_colleague2 = null;
         public function send($message, $colleague){
             if($colleague == $this->_colleague1){
                 $this->_colleague2->notify($message);
             }else{
                 $this->_colleague1->notify($message);
             }
         }

         public function set($colleague1, $colleague2){
             $this->_colleague1 = $colleague1;
             $this->_colleague2 = $colleague2;
         }
     }

     class Colleague1 extends Colleague{
         public function notify($message){
             echo 'Colleague1 recv :'.$message;
         }
     }

     class Colleague2 extends Colleague{
         public function notify($message){
             echo 'Colleague2 recv: '.$message;
         }
     }

     $mediator = new ConcreteMediator();
     $objC1 = new Colleague1($mediator);
     $objC2 = new Colleague2($mediator);

     $mediator->set($objC1,$objC2);

     $objC1->send('to c2 from c1');
     echo '<br>';
     $objC2->send('to c1 from c2');
 ?>