<?php
$username = 'administrator';
$apiKey = '7IjRKva9OzwCsy9xaCwsihA6HxUTwE7a_999';
$localUrl = 'https://localhost';

$messageId = 35;
$url = $localUrl . '/api/2.0/emails/' . $messageId;
$method = 'DELETE';
$auth = base64_encode($username . ':' . $apiKey);

$cSession = curl_init();

curl_setopt($cSession, CURLOPT_URL, $url);
curl_setopt($cSession, CURLOPT_RETURNTRANSFER, true);
curl_setopt($cSession, CURLOPT_HEADER, false);
curl_setopt($cSession, CURLOPT_CUSTOMREQUEST, strtoupper($method));
curl_setopt($cSession, CURLOPT_POSTFIELDS, '{}');
curl_setopt($cSession, CURLOPT_HTTPHEADER, [
    'Authorization: Basic ' . $auth,
    'Content-Type: application/json'
]);

$response = curl_exec($cSession);
var_dump($response);

curl_close($cSession);