Starred username

<?php

$username = "JohnnyNoxville";
$num = strlen($username) / 3;

$i = 0;
$starredname = "";

while($i < $num) {
    $starredname .= $username[$i];

    $i++;
}

while($i < $num * 2) {
    $starredname .= "*";

    $i++;
}

while($i < strlen($username)) {
    $starredname .= $username[$i];

    $i++;
}

echo $starredname;

?>