Have you tried CURL?
I usually use CURL to make API requests.
$url = ' http://web2sms.cosmote.gr/web2sms/DCM';
$fields = array(
'U' => urlencode($_POST['Username']),
'P' => urlencode($_POST['Password']),
'A' => urlencode($_POST['WEB2SMS']),
'U' => urlencode($_POST['username']),
'ACCOUNT' => urlencode($_POST['AccountName']),
'L' => urlencode($_POST['Language'])
);
//Convert the data into a value URI
foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
rtrim($fields_string, '&');
//open connection
$ch = curl_init();
//set the url, number of POST vars, POST data
curl_setopt($ch,CURLOPT_URL, $url);
curl_setopt($ch,CURLOPT_POST, count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);
//execute post
$result = curl_exec($ch);
//close connection
curl_close($ch);