fluent expressions and conditionals

One problem I have encountered when designing "dsl" fluent expressions in php is conditionals.

If you are chaining methods and you have a method a which could fail and thus affect method b it is best to execute them independently and check the validity with an if clause before calling method b.

class Chain{
  var $valid;
  var $__methods;
  var $__class_name;

  function __construct(){
    $this->valid = true;
    $this->__class_name = get_class($this);
    $this->__methods = get_class_methods($this->__class_name);
  }

  function __get($arg){
    if(in_array($arg,$this->__methods)){
      return $this->$arg();
    }
  }

  function __call($arg,$data){
    if(!$this->valid){
      return $this;
    }
    $arg = '__'.$arg;
    if(in_array($arg,$this->__methods)){
      return call_user_func_array(Array($this,$arg),$data);
    }
  }

  function __if($arg){
    $this->valid = $arg;
    return $this;
  }

  function __method1($value){
    echo $value."\n";
    return $this;
  }

  function start(){
    $this->valid = true;
    return $this;
  }

  function stop(){
    $this->valid = false;
    return $this;
  }
}

function chain(){
  $new = new Chain();
  return $new;
}
chain()->if(false)->method1('will not display');

$x = 10;
chain()->if($x == 10)
       ->stop
       ->method1('wont work')
       ->if(false)
       ->start
       ->method1('will not work - will now')
       ->if(false)
       ->method1('will not work either');