According to https://bugs.php.net/bug.php?id=61141:

On Windows setups using libcurl version 7.24 or later (which seems to correspond to PHP 5.3.10 or later), you may find that this always returns -1. This is, apparently, not strictly a bug: according to the libcurl documentation, you should add your own sleep if curl_multi_select returns -1.

For example:
<?php
/* setup $mh */

$active = null;
do {
    $mrc = curl_multi_exec($mh, $active);
} while ($mrc == CURLM_CALL_MULTI_PERFORM);

while ($active && $mrc == CURLM_OK) {
    if (curl_multi_select($mh) == -1) {
        usleep(100);
    }
    do {
        $mrc = curl_multi_exec($mh, $active);
    } while ($mrc == CURLM_CALL_MULTI_PERFORM);
}

/* close $mh */
?>