Zkuste něco takového :
$phoneNumber = $_POST['mobileno'];
if(!empty($phoneNumber)) // phone number is not empty
{
if(preg_match('/^\d{10}$/',$phoneNumber)) // phone number is valid
{
$phoneNumber = '0' . $phoneNumber;
// your other code here
}
else // phone number is not valid
{
echo 'Phone number invalid !';
}
}
else // phone number is empty
{
echo 'You must provid a phone number !';
}