public function shift() {
        if (empty($this->output)) {
            $this->output = array_reverse($this->input);
            $this->input = array();
        }
        if (empty($this->output)) {
            return NULL;
        }
        return array_pop($this->output);
    }
public function shift() {
        if (empty($this->output)) {
            $this->output = array_reverse($this->input);
            $this->input = array();
        }
        if (empty($this->output)) {
            return NULL;
        }
        return array_pop($this->output);
    }
public function shift ( ) { if ( empty ( $ this -> output ) ) { $ this -> output = array_reverse ( $ this -> input ) ; $ this -> input = array ( ) ; } if ( empty ( $ this -> output ) ) { return NULL ; } return array_pop ( $ this -> output ) ; }
Shifts an element off the front of the queue.
Shifts an element off the front of the queue .