To be thorough, you’ll want to start with parse_url().
$parts=parse_url("http://example.com/user/100");
That will give you an array with a handful of keys. The one you are looking for is path
.
Split the path on /
and take the last one.
$path_parts=explode("https://stackoverflow.com/", $parts['path']);
Your ID is now in $path_parts[count($path_parts)-1]
.