Get most viewed clips of the week

[BarryCarlyon]

You need to url encode that, as a + is a space. so you have to URL encode the + to %2b

$handle = curl_init('https://api.Twitch.tv/helix/clips?broadcaster_id=XXX&started_at='.$date);

becomes

$handle = curl_init('https://api.Twitch.tv/helix/clips?broadcaster_id=XXX&started_at='.urlencode($date));

And

$date = date('Y-m-d\TH:i:sP', strtotime('2020-05-11 00:00:00'));

You need the RFC3339 format

https://www.php.net/manual/en/class.datetime.php

Reference