if(isset($_POST['kirit'])){

    $matn = $_POST['matn'];

    $all = strlen($matn);

    echo'<h2>'.$all.' belgi</h2>';

}

echo'<form name="form" action="" method="POST">

        Matnni kiriting: <br>

        <textarea name="matn"></textarea><br>

        <input name="kirit" type="submit" value="Sanash">

</form>';