Apple pay merchant validation fails without key file

Last month our certificate key expired for Apple Pay, For generating new certificate

Followed this document TN3103: Apple Pay on the Web troubleshooting guide | Apple Developer Documentation )

1. My payment service provider created certificate signing request (CSR).(Not provided key)
2. Uploaded this CSR to the Merchant Configuration page to generate a Payment Processing Certificate
3. Downloaded payment processing certificate
4. Downloaded Merchant Identity Certificate

Both certificated installed in key chain, But the key file is not found so can't create Certificate.key.pem

`

public function apple_pay_session(Request $request)

   {  $certificatePassphrase  = "";
    $merchantIdentifier = "merchant.name.com";
    $displayName = "merchant name";
    $initiative = "web";
    $initiativeContext = "www.merchant.co.uk";
    $certPass = "";
    $certFile = public_path('merchant_id.pem');
    $keyFile = public_path('Certificate.key.pem');
   
    $data = '{"merchantIdentifier":"'.$merchantIdentifier.'","displayName":"'.$displayName.'","initiative":"'.$initiative.'","initiativeContext":"'.$initiativeContext.'"}';
    try {
        $ch1 = curl_init();
        Storage::put('computop_applpay_try_session_'.time().'.txt', "In session");
        // Check if initialization had gone wrong*    
        if ($ch1 === false) {
            throw new Exception('failed to initialize');
        }
        
        curl_setopt($ch1, CURLOPT_URL, $request->validationUrl);
        curl_setopt($ch1, CURLOPT_SSLCERT, $certFile);
        curl_setopt($ch1, CURLOPT_SSLKEY, $keyFile);
        curl_setopt($ch1, CURLOPT_SSLKEYPASSWD, $certPass);
        curl_setopt($ch1, CURLOPT_POST, 1);
        curl_setopt($ch1, CURLOPT_POSTFIELDS,$data);
        curl_setopt($ch1, CURLOPT_RETURNTRANSFER, 1);
        $merchantSession = curl_exec($ch1);
        
        // Check the return value of curl_exec(), too
        if ($merchantSession === false) {
            throw new Exception(curl_error($ch1), curl_errno($ch1));
        }
            
        return response()->json([
            "merchantSession"=>$merchantSession,
            "certFile" => $certFile
        ]);
    } catch(Exception $e) {
        Storage::put('computop_applpay_catch_session_'.time().'.txt', $e->getMessage());
        return response()->json([
            "code"=>$e->getCode(),
            "msg" => $e->getMessage()
        ]);
    }
}`

We only have apple_pay.cer ,merchant_id.cer, certificate signing request (CSR) provide by the payment service provider."

How to create merchant_id.pem,Certificate.key.pem from It..?

Or is it possible to do payment validation in PHP without these files?