How Do I Parse Result From curl_exec to extract json data in PHP

Have you tried:

curl_setopt($s,CURLOPT_HEADER,false); 

Basically, what you are receiving is a complete response from a server:

# these are the headers
RESPONSE: HTTP/1.1 400 Bad Request
Server: nginx
Date: Sat, 10 Jan 2015 17:31:02 GMT
Content-Type: application/json
Content-Length: 25
Connection: keep-alive
Keep-Alive: timeout=10

# This is the body.
{"error":"invalid_grant"}

By telling cURL to ignore the headers, you should only get {"error":"invalid_grant"}

Now, all of that said, the header separates the body by two newlines. So you should also be able to parse it that way:

$val = curl_exec();

// list($header,$body) = explode("\n\n", $val); won't work: \n\n is a valid value for 
// body, so we only care about the first instance.
$header = substr($val, 0, strpos($val, "\n\n"));
$body = substr($val, strpos($val, "\n\n") + 2);
// You *could* use list($header,$body) = preg_split("#\n\n#", $val, 2); because that
// will create an array of two elements.

// To get the value of *error*, you then
$msg = json_decode($body);
$error = $msg->error;

/*
 The following are because you asked how to "change the value of `error`".
 You can safely ignore if you don't want to put it back together.
*/
// To set the value of the error:
$msg->error = 'Too many cats!';

// to put everything back together:
$replaced = $header . "\n\n" . json_encode($msg);