ctype_space不识别仅空白字符串

Question

如果我只输入空白字符，查询仍会执行：

$newdisplayname = $_POST['newdisplayname'];
$sessionid = $_SESSION['userid'];

if(!empty($_POST) && isset($_SESSION['userid'])){

    if(strlen($newdisplayname)>=2 && strlen($newdisplayname)<=15 && !ctype_space($newdisplayname) && substr_count(strtoupper($newdisplayname), 'M') < 7 && substr_count(strtoupper($newdisplayname), 'W') < 7){

        // update displayname

        $stmt = $conn->prepare("UPDATE users SET u_displayname=? WHERE u_id=?");
        $stmt->bind_param("si", $newdisplayname, $sessionid);
        $stmt->execute();
        $stmt->close();

        echo "success";

    }

}

另外，如果我输入一个空格，它也会执行查询 - 它如何绕过strlen($newdisplayname)>=2？