php 创建随机字符串

function create_rand($length = 8, $type = 'all')
{
    $num = '0123456789';
    $letter = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    if ($type == 'num') {
        $chars = $num;
    } elseif ($type == 'letter') {
        $chars = $letter;
    } else {
        $chars = $letter . $num;
    }

    $str = '';
    for ($i = 0; $i < $length; $i++) {
        $str .= $chars[mt_rand(0, strlen($chars) - 1)];
    }
    return $str;

}