How do I fix this Bitly oAuth 'single header' error? - php

Question

Context: I have a WordPress plugin that allows users to authenticate with Bitly so that they can use link shortening features if they desire to do so.
Bitly requires one callback URL to be stored in the app settings, so I created a standalone script on my server that processes the authentication process. The overall handshake actually looks something like this:
The user clicks a link taking them to the Bitly authorization page for my app.
If they agree, they are forwarded to my script with a code.
My script exchanges that code to Bitly for another code.
Once the codes are all acquired, it reaches out to the user's domain to store the codes and to retreive the URL of the options page on that user's URL so that we can redirect the user back home.
We then redirect the user back to the options page that they had just clicked away from.
Problem: However, a very small handful of users are getting the following error just prior to the authentication process being complete:
Warning: Header may not contain more than a single header, new line
detected in /home/warfarep/public_html/bitly_oauth.php on line 72.
Sample Code:
function sw_file_get_contents_curl($url){
$ch=curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_USERAGENT, $_SERVER['HTTP_USER_AGENT']);
curl_setopt($ch, CURLOPT_FAILONERROR, 0);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($ch, CURLOPT_TIMEOUT, 5);
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
curl_setopt($ch, CURLOPT_NOSIGNAL, 1);
curl_setopt($ch, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4);
$cont = #curl_exec($ch);
$curl_errno = curl_errno($ch);
curl_close($ch);
if ($curl_errno > 0) {
return 0;
}
return $cont;
}
// Check if this is the first pass and we have the generic code
if(isset($_GET['code'])):
// Retreive the information
$code = $_GET['code'];
$client_ID = 'blahblahblah';
$client_secret = 'blahblahblah';
$redirect_url = 'blahblahblah';
$state = $_GET['state'];
$url = 'https://api-ssl.bitly.com/oauth/access_token';
$fields = array(
'code' => urlencode($code),
'client_id' => urlencode($client_ID),
'client_secret' => urlencode($client_secret),
'redirect_uri' => urlencode($redirect_url),
'state' => urlencode($state)
);
//url-ify the data for the POST
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, $fields_string);
curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch,CURLOPT_HTTPHEADER, array('Accept: application/json'));
//execute post
$response = curl_exec($ch);
//close connection
curl_close($ch);
$response = json_decode($response , true);
$ajax_url = $state .= '?action=sw_bitly_oauth&access_token='.$response['access_token'].'&login='.$response['login'];
$wp_response = sw_file_get_contents_curl($ajax_url);
$wp_response = rtrim($wp_response , '0');
header('Location: '.$wp_response);
^^^^^^ This is line 72 from the error message
endif;
Question: Is there anything that you can see that would prevent headers from already being sent prior to the redirect on line 72? How do I fix this Bitly oauth "single header" error? Thanks!