访问外部服务器上的文件

Is there any way to access files of Third party server using PHP?

Yes. You just fopen them if url_fopen is enabled, or use CURL.

The easiest way - assuming url_fopen_wrappers are enabled - is simply using file_get_contents() with a remote (http://, ftp://) URL.

If you don't want to rely on them being enabled, use CURL - while it requires a PHP extension it's pretty common so chances are high it's enabled even on shared hosting.

Here are examples for both methods:

// using url_fopen_wrappers
$contents = file_get_contents('http://stackoverflow.com');

// using CURL
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, 'http://stackoverflow.com');
curl_setopt($curl, CURLOPT_HEADER, false);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
$contents = curl_exec($curl);
curl_close($curl);

you could even use copy('thirdPartyFileUrl', 'fileO')