How to send a GET request from PHP?

Unless you need more than just the contents of the file, you could use file_get_contents.

$xml = file_get_contents("http://www.example.com/file.xml");

For anything more complex, I'd use cURL.

For more advanced GET/POST requests, you can install the CURL library (http://us3.php.net/curl):

$ch = curl_init("REMOTE XML FILE URL GOES HERE"); // such as http://example.com/example.xml
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HEADER, 0);
$data = curl_exec($ch);
curl_close($ch);

http_get should do the trick. The advantages of http_get over file_get_contents include the ability to view HTTP headers, access request details, and control the connection timeout.

$response = http_get("http://www.example.com/file.xml");

Guzzle is a very well known library which makes it extremely easy to do all sorts of HTTP calls. See https://github.com/guzzle/guzzle. Install with composer require guzzlehttp/guzzle and run composer install. Now code below is enough for a http get call.

$client = new \GuzzleHttp\Client();
$response = $client->get('https://example.com/path/to/resource');

echo $response->getStatusCode();
echo $response->getBody();

Depending on whether your php setup allows fopen on URLs, you could also simply fopen the url with the get arguments in the string (such as http://example.com?variable=value )

Edit: Re-reading the question I'm not certain whether you're looking to pass variables or not - if you're not you can simply send the fopen request containg http://example.com/filename.xml - feel free to ignore the variable=value part