Send/Post Form Data to Url with PHP

I have a form that submits via POST and I capture the variables once the form is submitted.

How can I concatenate the form data and then POST it to the url then re-direct to the thank you page?

This is not the exact code, I just can't find any normal answers, and I'm sure there is more than one way to do this. Just trying to figure out the simplest way possible.

if(isset($_POST['submit'])){
    $var1 = $_POST['var1'];
    $var2 = $_POST['var2'];

$url = 'https://api.this.com/foo/bar?token=IHAVETOKEN&foo=$Var1&bar=$var2'

post_request_to($url);

header("Location: thankyou.php");
}

EDIT:

HERE IS THE ACTUAL ANSWER/WORKING CODE:

if(isset($_GET['submit'])){
  $firstName = $_GET['firstname'];
  $lastName = $_GET['lastname'];
  $email = $_GET['email'];
  $password = $_GET['password'];
  $phone = $_GET['phone'];
}

  $data = array(
        'token' => 'sadfhjka;sdfhj;asdfjh;hadfshj',
        'firstName' => $firstName,
        'lastName' => $lastName,
        'email' => $email,
        'password' => $password,
        'phone' => $phone

    );


  $postvars = http_build_query($data) . "\n";

  $ch = curl_init();
  curl_setopt($ch, CURLOPT_URL, 'https://api.com/foo/bar?');
  curl_setopt($ch, CURLOPT_POST, 1);
  curl_setopt($ch, CURLOPT_POSTFIELDS, $postvars);

  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

  $server_output = curl_exec ($ch);

  curl_close ($ch);