PHP cURL CURLOPT_HEADERFUNCTION failing with cURL code 23: Failed writing header

Question

I'm attempting to use the PHP cURL lib to make a simple call to an API to retrieve data. I want to be able to store the response headers as well as the response body, and to do this I am using CURLOPT_HEADERFUNCTION as follows:

$headers = [];

curl_setopt(
    $cURLHandle,
    CURLOPT_HEADERFUNCTION,
    function ($cURLHandle, $header) use (&$headers) {
        $pieces = explode(":", $header);
        if (count($pieces) >= 2) $headers[trim($pieces[0])] = trim($pieces[1]);
    }
);

But when I run the above code, it produces cURL error #23: Failed writing header.

Why is this happening and how can I resolve this?

Answer 1

This is happening because the header length (in bytes) needs to be returned from the function you provide for CURLOPT_HEADERFUNCTION.

I believe, although I am not sure of this, this is because PHP needs/wants to be able to report the length of the headers from curl_getinfo(...) calls.

The fixed code would look something like the following:

$headers = [];

curl_setopt(
    $cURLHandle,
    CURLOPT_HEADERFUNCTION,
    function ($cURLHandle, $header) use (&$headers) {
        $pieces = explode(":", $header);
        if (count($pieces) >= 2) $headers[trim($pieces[0])] = trim($pieces[1]);
        return strlen($header); // <-- this is the important line!
    }
);