PHP固定长度字符串

/**
 * 获取固定长度随机字符串
 * @param $n
 * @return string
 * @throws Exception
 */
function gf_rand_str($n) {
    if (!is_int($n)) {
        throw new Exception(‘argument must be int‘);
    }
    $alpha = ‘ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789‘;
    $str = ‘‘;
    for ($i=0; $i<$n; $i++) {
        $str .= $alpha[rand(0, 35)];
    }
    return $str;
}

function invite_num($len = 6)
{
    $en_chars = [
        "A", "B", "C", "D", "E", "F", "G",
        "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R",
        "S", "T", "U", "V", "W", "X", "Y", "Z"
    ];
    $num_chars = [
        "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"
    ];
    $en_shuf = shuffle($en_chars);    // 将数组打乱
    $num_shuf = shuffle($num_chars);
    $output = "";
    for ($i = 0; $i < 3; $i++) {
        $output .= $en_chars[mt_rand(0, $len)];
    }
    $output .= substr(getMicroSecondsTimestamp(), 11, 1);
    $output .= rand(10, 99);
    return $output;
}
function getMicroSecondsTimestamp()
{
    $time = microtime();
    return substr($time, 11, 10) . str_pad(substr($time, 0, 8) * 1000000,
            6, "0", STR_PAD_LEFT);
}