https://your_server/api/trackedEntityInstances/CM1zLmk6qvA?strategy=DELETE

$href  = $request->get('href');

$url = $href."?strategy=DELETE";
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt( $ch, CURLOPT_HTTPHEADER, array('Content-Type:application/json'));
curl_setopt( $ch, CURLOPT_USERPWD, "user:pass");	
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "DELETE");
$result = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);