imap_getmailboxes() is undefined

### HHVM Version

3.15
### Standalone code, or other way to reproduce the problem

$mbox = imap_open("{imap.example.org}", "username", "password", OP_HALFOPEN)
or die("can't connect: " . imap_last_error());

$list = imap_getmailboxes($mbox, "{imap.example.org}", "*");
if (is_array($list)) {
foreach ($list as $key => $val) {
echo "($key) ";
echo imap_utf7_decode($val->name) . ",";
echo "'" . $val->delimiter . "',";
echo $val->attributes . "
\n";
}
} else {
echo "imap_getmailboxes failed: " . imap_last_error() . "\n";
}

imap_close($mbox);
### Expected result

$resource array
### Actual result

Fatal error: Call to undefined function Ddeboer\Imap\imap_getmailboxes() in /data/sites/www.sproutconnections.com/libraries/Ddeboer/Imap/Connection.php on line 244