I want to implement a funtionallity that user can login to their dropbox account by my web application and i want to show all folder of dropbox on my web application screen . How can i do this .

Now am using

CURLOPT_HTTPHEADER => array( 'Authorization: Bearer ' . $key, 'Dropbox-API-Arg:' . $data),

but it giving me below error.

Error in call to API function "files/download": You provided a non-empty HTTP "Content-Type" header ("application/x-www-form-urlencoded"). This API function requires that the header be missing or empty.

Can you please tell me what parameter should i pass to get response

Sorry, I think I steered you wrong. Your Content-Type header was fine... here's code that's working for me:

$accessToken = '<REDACTED>';

$ch = curl_init();
$data = json_encode(array('path' => '/test.txt'));
curl_setopt_array($ch, array(
    CURLOPT_URL => 'https://content.dropboxapi.com/2/files/download',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_POST => true,
    CURLOPT_HTTPHEADER => array("Content-Type:", "Authorization: Bearer $accessToken", "Dropbox-API-Arg: $data"),
));
$output = curl_exec($ch);
curl_close($ch);
echo $output;

An alternative (drop both CURLOPT_POST and the Content-Type header):

curl_setopt_array($ch, array(
    CURLOPT_URL => 'https://content.dropboxapi.com/2/files/download',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_CUSTOMREQUEST => "POST",
    CURLOPT_HTTPHEADER => array("Authorization: Bearer $accessToken", "Dropbox-API-Arg: $data"),
));

Now am using above code. Error is removed now but its not returning any response.

If you're not getting an error, the response should be the file content. Is it possible the file is 0 bytes? You can check on the Dropbox web site.