CURLOPT_PROXY_SSLCERT

CURLOPT_PROXY_SSLCERT is one of the accepted parameters for curl_setopt parameter #2.

CURLOPT_PROXY_SSLCERT — The file name of your client certificate used to connect to the HTTPS proxy. The default format is "P12" on Secure Transport and "PEM" on other engines, and can be changed with CURLOPT_PROXY_SSLCERTTYPE. With NSS or Secure Transport, this can also be the nickname of the certificate you wish to authenticate with as it is named in the security database. If you want to use a file from the current directory, please precede it with "./" prefix, in order to avoid confusion with a nickname. Available as of PHP 7.3.0 and cURL 7.52.0

CURLOPT_PROXY_SSLCERT = 10254;