PHP随机数怎么字母加数字组合，前两个是字母后边是数字？

function getRandomString($prefixLength = 2, $suffixLength = 6) {
    $t = '';
    for ($i=0; $i<$prefixLength; $i++) {
        $salt = mt_rand(1000, 9999);
        if ($salt % 2) {
            $t .= chr(mt_rand(97,122));
        } else {
            $t .= chr(mt_rand(65,90));
        }
    }
    $min = intval(str_pad('1', $suffixLength, '0', STR_PAD_RIGHT));
    $max = intval(str_repeat('9', $suffixLength));
    return $t . mt_rand($min, $max);
}

function randomKey(){
    $re = '';
    $letter = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    while(strlen($re)<2) {
        $re .= $letter[rand(0, strlen($letter)-1)]; /** 从$letter中随机产生一个字符 */
    }
    $_rand = mt_rand(10000000, 99999999); /** 生成8位随机数 */
    list($usec, $sec) = explode(" ", microtime()); /** 获取微秒时间戳 */
    $microtime = str_replace(".", "", ((float)$usec + (float)$sec)); /** 处理字符串 */
    
    $result = $re.$_rand.$microtime;
}