Unable to set private key file ?

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 ?