what to use instead of the http()?

There seems to be no function http() on my system. What can I use instead of http()? 
    $response = http($metadata->token_endpoint, [
        'grant_type' => 'authorization_code',
        'code' => $_GET['code'],
        'redirect_uri' => $redirect_uri,
        'client_id' => $client_id,
        'client_secret' => $client_secret,
    ]);

proof that an old dog can learn new tricks

Turns out that the http() is a function that was left out of the code.

proof that an old dog can learn new tricks