Very simply:
$id = substr($url, strrpos($url, '/') + 1);
strrpos gets the position of the last occurrence of the slash; substr returns everything after that position.
As mentioned by redanimalwar if there is no slash this doesn't work correctly since strrpos
returns false. Here's a more robust version:
$pos = strrpos($url, '/');
$id = $pos === false ? $url : substr($url, $pos + 1);
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