/**
 * 产生随机字符串
 *
 * @param    int        $length  输出长度
 * @param    string     $chars   可选的 ，默认为 0123456789
 * @return   string     字符串
 */
function random($length, $chars = '0123456789') {
    $hash = '';
    $max = strlen($chars) - 1;
    mt_srand();
    for($i = 0; $i < $length; $i++) {
        $hash .= $chars[mt_rand(0, $max)];
    }
    return $hash;
}

$a=random(8, '1294567890abcdefghigklmnopqrstuvwxyzABCDEFGHIGKLMNOPQRSTUVWXYZ');

var_dump($a);///mnt/hgfs/www/test/index.php:28:string 'otFTNx9u' (length=8)