Рабочий пример кода:
$curl = curl_init();
curl_setopt_array($curl, array(
CURLOPT_URL => "https://api.instagram.com/oauth/access_token",
CURLOPT_RETURNTRANSFER => true,
CURLOPT_ENCODING => "",
CURLOPT_MAXREDIRS => 10,
CURLOPT_TIMEOUT => 0,
CURLOPT_FOLLOWLOCATION => true,
CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
CURLOPT_CUSTOMREQUEST => "POST",
CURLOPT_POSTFIELDS => array('client_id' => '{client_id}','client_secret' => '{client_secret}','grant_type' => 'authorization_code','redirect_uri' => '{redirect_uri}','code' => '{code}),
CURLOPT_HTTPHEADER => array(
"Content-Type: multipart/form-data; boundary=--------------------------780367731654051340650991"
),
));
$response = curl_exec($curl);
curl_close($curl);
echo $response;