The PHP configuration doesn't allow you to request URLs as files. This is probably for security.

see: http://phpsec.org/projects/phpsecinfo/tests/allow_url_fopen.html

and: http://us3.php.net/manual/en/filesystem.configuration.php#ini.allow-url-include

The two PHP settings that affect this are allow_url_fopen and allow_url_include. The two Joomla sites must have different settings affecting them. Use phpinfo() and ini_get() to figure out the difference.

allow_url_fopen enables/disables all remote file access for PHP.
allow_url_include enables/disables remote file execution (file access through include() or require())

The allow_url_include was introduced in 5.10 but allow_url_fopen since PHP4.

Not being able to do require() means either allow_url_include or allow_url_fopen is disabled, or both. Usually it would be just allow_url_include.

First you have to ask, do you really trust the URL: http://site.com/LinkRotate/showlink.php?id=2
Since you will be executing any PHP on that URL.

If you do, then you can use an alternative:

eval(file_get_contents('http://site.com/LinkRotate/showlink.php?id=2'));

Even if you fully trust site.com, this is still very dangerous. Anyone that can modify network traffic between site.com and your site can run arbitrary code on your site.

If allow_url_fopen is also turned off, then you'll have to use CURL, or other HTTP client, if available. Or ask the host if they could change the PHP.ini settings, or you could change it for your specific site. Some sites even allow you to have your custom built PHP binary.