Generate an Authentication Code in PHP

In some email activation and verification processes we would need to make authentication codes for those verifications. This PHP function helps in generating such authentication codes for verification purpose. Make use of this code snippet for generating authentication codes.

function generateAuthenticationCode($length) {
    $ac = "abcdefghijklmnopqrstuvwxyz0123456789";
    $ac_len = strlen($ac);
    for($i=0; $i<$length; $i++){
        $position = rand(0,$ac_len);
        $authenticationCode .= $ac{$position};
    }
    return $authenticationCode;
}