PHP面对对象抽象类练习

<?php abstract class Traffic{     abstract public function move($foot);    public function color($col){        echo "The traffic is ".$col."\n";        }}        class car extends Traffic{            public function move($foot){                echo "the car moves ".$foot." m."."\n";                }        }        class train extends Traffic{            public function move($foot){                echo "the train moves ".$foot." m."."\n";            }        }        $cars = new car();        $trains = new train();        $cars->move("20");        $cars->color("red");        $trains->move("50");        $trains->color("green");?>