阿里云服务器随机数生成 结果为什么固定

<code>public function randStr($len = 6, $format = 'ALL') {
    switch ($format) {
        case 'ALL':
            $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-@#~';
            break;
        case 'CHAR':
            $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-@#~';
            break;
        case 'NUMBER':
            $chars = '0123456789';
            break;
        default :
            $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-@#~';
            break;
    }
    mt_srand((double) microtime() * 1000000 * getmypid());
    $password = "";
    while (strlen($password) </code>

<code>public function randStr($len = 6, $format = 'ALL') {
    switch ($format) {
        case 'ALL':
            $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-@#~';
            break;
        case 'CHAR':
            $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-@#~';
            break;
        case 'NUMBER':
            $chars = '0123456789';
            break;
        default :
            $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-@#~';
            break;
    }
    mt_srand((double) microtime() * 1000000 * getmypid());
    $password = "";
    while (strlen($password) </code>