<?php
$whmcsUrl = "http://srkhost.eu/";

$username = "username";
$password = "password";

$postfields = array (
    'username' => $username,
    'password' => $password,
    'action' => 'GetClients',
    'responsetype' => 'json',
);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $whmcsUrl . 'includes/api.php');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($postfields));
$response = curl_exec($ch);
if (curl_error($ch))
{
    die('Unable to connect: ' . curl_errno($ch) . ' - ' . curl_error($ch));
}
curl_close($ch);
$jsonData = json_decode($response, true);
echo '<pre>';
var_dump($jsonData);
?>