Create an alternative fetcher for aggregator.module.
A fetcher downloads feed data to a Drupal site. The fetcher is called at the first of the three aggregation stages: first, data is downloaded by the active fetcher; second, it is converted to a common format by the active parser; and finally, it is passed to all active processors, which manipulate or store the data.
Modules that define this hook can be set as the active fetcher within the configuration page. Only one fetcher can be active at a time.
$feed: A feed object representing the resource to be downloaded. $feed->url contains the link to the feed. Download the data at the URL and expose it to other modules by attaching it to $feed->source_string.
TRUE if fetching was successful, FALSE otherwise.
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_aggregator_fetch($feed) {
$feed->source_string = mymodule_fetch($feed->url);
}