Perhaps try executing it like this:
<?php
$url = 'http://bulksms.mysmsmantra.com/WebSMS/SMSAPI.php';
$fields = array(
'username' => "hidden",
'password' => "hidden",
'sendername' => "iZycon",
'mobileno' => 8443223
);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, count($fields));
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($fields));
$result = curl_exec($ch);
curl_close($ch);
var_dump($result);
source
share