Before PHP 7.1.23 and 7.2.11, curl_multi_select() would return -1 if cURL had no open file descriptors. Since then, assuming libcurl 7.28 or higher, curl_multi_select() will return 0 if cURL has no open file descriptors and -1 on error.
See https://bugs.php.net/bug.php?id=76480
This is a BC break. See https://bugs.php.net/bug.php?id=77030