$cost = 0;
$body = 'Test';
$rid = '94547377';
$rtype = 'characters';
$sub = 'test object';
$auth_code = 'my authcode';

function curl_post_group()
{
    $ch = curl_init();
    $url = 'https://esi.tech.ccp.is/latest/characters/94547377/mail/?datasource=tranquility';
    curl_setopt($ch, CURLOPT_URL, $url);
    // Set so curl_exec returns the result instead of outputting it.
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    // Does not verify peer
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    // Get the response and close the channel.
    $headers = array(
        "Authorization: Bearer " . $auth_code,
        "Content-type: json"
    );
    // headers
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    // Post options, for get comment out
    curl_setopt($ch, CURLOPT_POST, 1);
    $post ["approved_cost"] = $cost;
    $post ["body"] = $body;
    $post ["recipients"] = array();
    $post ["recipients"] ["recipient_id"] = $rid;
    $post ["recipients"] ["recipient_type"] = $rtype;
    //$post ["recipient_id"] = $rid;
    //$post ["recipient_type"] = $rtype;
    $post ["subject"] = $sub;
    $post = json_encode($post, JSON_UNESCAPED_UNICODE);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
    curl_setopt($ch, CURLOPT_FRESH_CONNECT, true);
    $response = curl_exec($ch);
  $response = curl_exec($ch);
    if (curl_getinfo($ch) ["http_code"] != 201) {
        echo "Error";
        echo " HTTP_CODE:" . curl_getinfo($ch) ["http_code"];
        print_r(curl_getinfo($ch));
        print_r($response);
    }
    curl_close($ch);
    $response = json_decode($response);
    return $response;
}
curl_post_group();

curl -i --compressed -X POST --header 'Content-Type: application/json' --header 'Accept: application/json' --header 'Authorization: Bearer 'auth code from geuthur' -d '{ \ 
   "approved_cost": 0, \ 
   "body": "string", \ 
   "recipients": [ \ 
     { \ 
       "recipient_id": 0, \ 
       "recipient_type": "alliance" \ 
     } \ 
   ], \ 
   "subject": "string" \ 
 }' 'https://esi.tech.ccp.is/latest/characters/94547377/mail/?datasource=tranquility'

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Project Page: https://github.com/eveseat/seat