- preg_replace: documentation ( source)
<?php
$sPhoneNumber = "+3999999999999";
$sPhoneNumber = "3999999999999";
$sPhoneNumber = "39 99999999999";
$sPhoneNumber = "0039 99999999999";
$sPhoneNumber = "+ 39 999 99999999";
$sPhoneNumber = "+003999999999999";
$result = preg_replace('~^[0\D]++|\D++~', '', $sPhoneNumber);
echo $result;