create random string

Hello, I have a function which is supposed to create a random 8 character string, it works most of the time, but sometimes it creates a 7 character string.. very weird, please help...

function genRandomString() {
    $length = 8;
    $characters = 'ABCDEFGHJKLMNPQRSTUVWXYZ23456789';
    $string = "";    
    for ($p = 0; $p < $length; $p++) {
        $string .= $characters[mt_rand(0, strlen($characters))];
    }
    return $string;
}