How to set content type in php curl?

I need to send a post request to a website using php curl. I set header Content-type = application/json but curl_info still shows Content-type = null.

$url = "https://api.business.kazanexpress.ru/api/oauth/token";

$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);

$headers = array(
'Content-Type: application/json',
"Accept: application/json",
);
curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
var_dump(curl_getinfo($curl));