generate random number and catcha code in php

function Random()
{
$chars = "ABCDEFGHJKLMNPQRSTUVWZYZ7589641230";
srand((double)microtime()*1000000);
$i = 0;
$pass = '' ;
while ($i <= 4)
{
$num = rand() % 32;
$tmp = substr($chars, $num, 1);
$pass = $pass . $tmp;
$i++;
}
return $pass;
}

?>