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>';