PHP Fatal error: Call to undefined function setMethod() [on hold]

You need to replace all your calls to class methods. From - to -> your missing > as PHP looks at it as $req minus the value of function setMethod(HTTP_REQUEST_METHOD_GET); which is not a function but rather a class method.

E.G:

echo "Inviting $invite_steam_id ($fid): ";
$req->setMethod(HTTP_REQUEST_METHOD_GET);
$req->setUrl($url);

$res = $req->sendRequest();
if (PEAR::isError($res))
die($res->getMessage());

Also you may need to look and fix the class to not rely on any reference passing as its Deprecated, it is not required and can be removed:

$req = &new HTTP_Request('https://steamcommunity.com');

Should be:

$req = new HTTP_Request('https://steamcommunity.com');

That will fix the Deprecated notices.