handle curl_close only for PHP < 8

[mrohnstock]

In PHP < 8, curl_init() returns a resource that must be explicitly closed using curl_close() to avoid memory leaks.

Starting with PHP 8, curl_init() returns a CurlHandle object which is automatically cleaned up by the garbage collector. Calling curl_close() on it has no effect and triggers a deprecation warning in PHP 8.5+.

This change ensures curl_close() is only called for resource handles.