har har but wait… this is what I got:
$url="https://xxx.mailify.de:xxx";
$apikey="xx-xxxxxxxxxx";
$call="/rest/system/connections";
$ch = curl_init($url."".$call);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_HTTPHEADER,array('X-API-Key: '.$apikey.''));
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST,0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER,0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$output = curl_exec($ch);
curl_close($ch);
echo "<pre>".$output."</pre>";