вы можете использовать cURL, например
$data = [
'username' => $username,
'email' => $email,
'password' => $password
];
//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($data));
curl_setopt($ch,CURLOPT_POSTFIELDS, http_build_query($data));
//So that curl_exec returns the contents of the cURL; rather than echoing it
curl_setopt($ch,CURLOPT_RETURNTRANSFER, true);
//execute post
$result = curl_exec($ch);
echo $result;