What is different between set opt CURLOPT_USERAGENT and set useragent in header field curl php?

Question

someone please help me know the difference about setting opt in curl php.

$curl = curl_init();
curl_setopt_array($curl, array(
  CURLOPT_URL => "abcxyz",
  CURLOPT_CUSTOMREQUEST => "GET",
  CURLOPT_HTTPHEADER => array(
    "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
    "Accept-Language: en-US,en;q=0.5",
    "Cache-Control: no-cache",
    "Connection: keep-alive",
    "Cookie: ht=7635aa7ceda60bf1",
    "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0"
  ),
));

and

$curl = curl_init();
    curl_setopt_array($curl, array(
      CURLOPT_URL => "abcxyz",
      CURLOPT_CUSTOMREQUEST => "GET",
      CURLOPT_COOKIE => "ht=7635aa7ceda60bf1",
      CURLOPT_USERAGENT => "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0",
      CURLOPT_HTTPHEADER => array(
        "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
        "Accept-Language: en-US,en;q=0.5",
        "Cache-Control: no-cache",
        "Connection: keep-alive"
      ),
    ));

I always get error when trying to use CURLOPT_COOKIE and CURLOPT_USERAGENT.

Answer 1

Ok, I did some actual testing and it seems like the 'User-Agent' specified inside CURLOPT_HTTPHEADER takes precedence over the value CURLOPT_USERAGENT.

Here is the test setup and results:

$curl = curl_init();
curl_setopt($curl, CURLOPT_USERAGENT, 'Curl');

curl_setopt_array($curl, array(
    CURLOPT_URL => "https://webhook.site/fdb48603-6fe3-48e1-a542-75bb62355522",
    CURLOPT_CUSTOMREQUEST => "GET",
    CURLOPT_HTTPHEADER => array(
        "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
        "Accept-Language: en-US,en;q=0.5",
        "Cache-Control: no-cache",
        "Connection: keep-alive",
        "Cookie: ht=7635aa7ceda60bf1",
        "User-Agent: Override"
    ),
));

$response = curl_exec($curl);
$err = curl_error($curl);

And here is the result:

I also changed the order but it doesn't matter. But if I comment out the User-Agent in CURLOPT_HTTPHEADER then the reported user-agent is 'Curl' (or the value specified with CURLOPT_USERAGENT)

So basically the value you specify in CURLOPT_USERAGENT can be thought of as the default value, and the value you put inside CURLOPT_HTTPHEADER is the way to override that default.