A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

class Delegator {
  function __construct() {
    $this->delegate = NULL ;
  }
  function operation() {
    if(method_exists($this->delegate, "thing"))
      return $this->delegate->thing() ;
    return 'default implementation' ;
  }
}
 
class Delegate {
  function thing() {
    return 'Delegate Implementation' ;
  }
}
 
$a = new Delegator() ;
print "{$a->operation()}\n" ;
 
$a->delegate = 'A delegate may be any object' ;
print "{$a->operation()}\n" ;
 
$a->delegate = new Delegate() ;
print "{$a->operation()}\n" ;