$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => 'https://app.wallmessage.com/api/create-message',
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => '',
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => 'POST',
  CURLOPT_POSTFIELDS => array(
  'appkey' => '2a083fca-e0cd-4899-89c1-8b8758a129e0',
  'authkey' => 'Vt747KVGqA2oxnn7arhzLHWEKCPoTBnL9pwxcaI7y3C8U5KTLB',
  'to' => 'RECEIVER_NUMBER',
  'message' => 'Example message',
  'sandbox' => 'false'
  ),
));

$response = curl_exec($curl);

curl_close($curl);
echo $response;