Some sites require additional work to be performed on their page URLs in order to extract a media ID that can be used in embedded HTML. This callback can be used to perform that work.<br />
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Your callback should return the unique ID of the media being requested, and have a signature as follows:<br />
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<code>public static function matchCallback($url, $matchedId, \XF\Entity\BbCodeMediaSite $site, $siteId)</code>