<script>

function updateM(){

if(document.getElementById("m").value.length<10){

alert("1234567899");

}

else{

document.updateMobile.submit();

}

}

</script>

<form action="updateMobile.php" method="post" name="updateMobile">

<p align="center">0123456

<input type="text" name="m" id="m" size="20" value="0555555555">

<input type=button value="تعديل" name="mobile" id="mobile" onclick="updateM();">

</p>

<input type=hidden name="id_user" value='1234'>

</form>