<?php
function random_username($string) {
	$pattern = " ";
	$string = strtolower($string);
	$strstr = strstr($string, $pattern, false);
	$randCount = rand(1, strlen($strstr));
	$parts = array(
		trim(strstr($string, $pattern, true)),
		trim(substr($strstr, 0, $randCount))
	);
	$nrRand = rand(0, 100);
	shuffle($parts);
	$username = $parts[0]. $parts[1] .trim($nrRand);
	return $username;
}

echo random_username("Luca Montanera");