$whmcsUrl = "https://test.etravelsmart.com/etsAPI/api/getStations?";
// Admin username and password
$username = "888888888";
$password = "**********";
// Set post values
$postfields = array(
    'username' => $username,
    'password' => md5($password),
    'action' => 'GetClients',
    'responsetype' => 'json',
);
$arrContextOptions=array(
    "ssl"=>array(
        "verify_peer"=>false,
        "verify_peer_name"=>false,
    ),
);

// Call the API
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $whmcsUrl);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($postfields));
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
$response = curl_exec($ch);
if (curl_error($ch)) {
    die('Unable to connect: ' . curl_errno($ch) . ' - ' . curl_error($ch));
}
curl_close($ch);
// Attempt to decode response as json
$jsonData = json_decode($response, true);
// Dump array structure for inspection
var_dump($jsonData);

{
    "stationList":[
        {"stationName":"10   dol","stationId":-1},
        {"stationName":"Chennai","stationId":2666}
    ],
    "apiStatus":{"success":true,"message":"SUCCESS"}
}