There's two problems here which lead to a blank screen. You used http instead of https in your $url. https is required for fitbit token request. You're also running json_decode on a string that's not a json array. The data inside $json is a string from http body response.
Here's my run down in detail to make it fully working below.
Instead of:
$url = "http://api.fitbit.com/oauth/request_token";
Use:
$url = "https://api.fitbit.com/oauth/request_token";
Also remove:
$fitbit_data = json_decode($json);
Change:
echo '<pre>'.print_r($json,true).'</pre>'; ?>
To:
echo '<pre>'. $json .'</pre>'; ?>
This will leave you with the successful response of:
<pre>oauth_token=93bf6ded129fcfa2b687ce6e625477af&oauth_token_secret=c9cc547ca6dd484f73763e23b0987121&oauth_callback_confirmed=true</pre>
shown into your browser.
P.S. make sure to remake this to Oauth 2.0. OAuth 1.0a support will be removed from the Fitbit Web API on April 12, 2016.