PHP Genera clave password random

<?
function createRandomPassword()
{
$chars = "123456";

srand((double)microtime()*1000000);
$i = 0;
$pass = '' ;

do
{
$num = rand() % 33;
$tmp = substr($chars, $num, 1);
$pass = $pass . $tmp;
$i++;
}
while (strlen($pass)!=1);

return $pass;
}
?>