$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => 'https://web.it-decision.com/v1/api/send-sms',
  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 => json_encode(array(
      "phone" => "380632132121",
      "sender" => "InfoItd",
      "text" => "This is messages DecisionTelecom",
      "validity_period" => 300
  )),
  CURLOPT_HTTPHEADER => array(
    'Authorization: Basic <your_base64_encoded_api_key>',
    'Content-Type: application/json',
  ),
));

$response = curl_exec($curl);

if (curl_errno($curl)) {
    echo 'Error:' . curl_error($curl);
}

curl_close($curl);
echo $response;