I have this PHP code to connect to apple pay (WEB): $validationPayload = json_encode([ 'merchantIdentifier' => "merchant.com.XXXXXX", 'displayName' => "XXXX", 'initiative' => "web", 'initiativeContext' => "my_domain.com" ]); $response = (new Client())->post('https://apple-pay-gateway-cert.apple.com/paymentservices/startSession', [ 'headers' => [ 'Content-Type' => 'application/json' ], 'body' => $validationPayload, 'cert' => storage_path('apple-pay/certificate.pem'), ]) ->getBody() ->getContents(); return $response; I got the cer file from the payment gateway and import it to my account and activate it. I uploaded the cer file in (Apple Pay Payment Processing Certificate) I converted this cer file to pem by this command: openssl x509 -inform DER -in apple_pay.cer -out certificate.pem When I call the code above I get this error: "cURL error 58: unable to set private key file: '/Users/a/Documents/Sites/ish/storage/apple-pay/certificate.pem' type PEM (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://apple-pay-gateway-cert.apple.com/paymentservices/startSession" So how can I convert the cer(that the payment gave to me by the gateway) to pem ?