How to substitute the proxy type from the database in curl?

Question

Parsing

This doesn't work:
curl_setopt($curl, CURLOPT_PROXY, "$proxy_type://$proxy_host:$proxy_port");
so too:
curl_setopt($curl, CURLOPT_PROXYTYPE, "CURLPROXY_$proxy_type");
$proxy_type - socks5 or http proxy type.