Quickstart: PHP

The cURL extension (enabled in virtually every PHP build) is all you need.

send.php

$ch = curl_init("https://api.calder.click/v1/emails");
curl_setopt_array($ch, [
    CURLOPT_POST => true,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_TIMEOUT => 10,
    // headers incl. Idempotency-Key, JSON body —
    // full source in the copy button above.
]);

Check curl_errno($ch) for transport failures and retry with the same key. Then continue to Webhooks.