PHP检索字符串（strstr）文件上传查找指定（.jpg）文件

<form method="post" action="index.php" enctype="multipart/form-data">    <input type="hidden" name="action" value="upload">    <input type="file" name="u_file">    <input type="submit" name="アップロート"></form><?php    if ($_POST[action]=="upload") {        $file_path="./uploads\\";        $picture_name=$_FILES[u_file][name];        $picture_name=strstr($picture_name, ".");        if ($picture_name!=".jpg") {            echo "                <script type='text/javascript'>                    alert('not jpg');                    windows.location.href='index.php';                </script>            ";        }elseif ($_FILES[u_file][tmp_name]) {            move_uploaded_file($_FILES[u_file][tmp_name], $file_path.$_FILES[u_file][name]);//图片上传            echo "upload success";        }else{            echo "upload fail";        }    }?>