用个方法 function Notice($alert,$url){

               echo "<script language='javascript'>";
         echo "alert('".$alert."');";
         echo "location.href='".$url."';";

                       echo "</script>";}
                                $username=$_POST['username'];   
                                if(empty($username))
                                    Notice("不能为空","test.php");

                                else{
                                    echo $username;