- strpos: documentation ( source)
- number_format: documentation ( source)
<?php
$number = '+421917982116';
$haveNumberPlus = strpos($number, '+');
if (!$haveNumberPlus) {
echo number_format((int)$number, 0, ' ', ' ');
}
echo '+' . number_format((int)$number, 0, ' ', ' ');