Server :: Apache - "file_get_contents" Works Only With Port 80?

I have a web site on port 80. My apache works on port 8080. i configured cache_module in apache. so that the clients will enter the web site threw the apache and apache will gather all the cache of the web site in order to speed up the service. when i point apache to take content from port 80 - it works just fine. when i change it from port 80 to any other port, it gives an error: "failed to open stream: Permission denied in /var/www/web/cache/index.php on line 44" it works fine with port 80. so how come not other port? i will be glad to hear any suggestion.