在 PHP 中正则表达式匹配规则的调整(允许某一个捕获子组为空)

// $va = 'internal://all_collectionss/';
$va = 'internal://all_collectionss/1';
$matches = [];
if (preg_match('#internal://([^/]+)s/([^/]+)#', $va, $matches)) {
	$resource = strtolower($matches[1]);
	$resourceId = $matches[2];
}

Match 1 0-29	internal://all_collectionss/1
Group 1 11-26	all_collections
Group 2 28-29	1

Your regular expression does not match the subject string.
Try launching the debugger to find out why.

// $va = 'internal://all_collectionss/';
$va = 'internal://all_collectionss/1';
$matches = [];
if (preg_match('#internal://([^/]+)s/([^/]*)#', $va, $matches)) {
	$resource = strtolower($matches[1]);
	$resourceId = $matches[2];
}