Gemini API via PHP cURL: InvalidSignature

I converted the following Python code from the official docs(https://docs.sandbox.gemini.com/rest-api/#private-api-invocation) to PHP but I always get InvalidSignature error:

url = "https://api.gemini.com/v1/mytrades"
gemini_api_key = "mykey"
gemini_api_secret = "1234abcd".encode()

t = datetime.datetime.now()
payload_nonce = time.time()
payload =  {"request": "/v1/mytrades", "nonce": payload_nonce}
encoded_payload = json.dumps(payload).encode()
b64 = base64.b64encode(encoded_payload)
signature = hmac.new(gemini_api_secret, b64, hashlib.sha384).hexdigest()

request_headers = {
    'Content-Type': "text/plain",
    'Content-Length': "0",
    'X-GEMINI-APIKEY': gemini_api_key,
    'X-GEMINI-PAYLOAD': b64,
    'X-GEMINI-SIGNATURE': signature,
    'Cache-Control': "no-cache"
    }

response = requests.post(url, headers=request_headers)

My PHP code is this and everything looks correct:

$b64 = base64_encode(utf8_encode(json_encode([ "request" => "/v1/balances", "nonce" => time() ])));
$header = [
        'Content-Type: text/plain',
        'Content-Length: 0',
        'X-GEMINI-APIKEY: master-XXXXXXX',
        'X-GEMINI-PAYLOAD: ' . $b64,
        'X-GEMINI-SIGNATURE: ' . md5(hash_hmac('sha384', utf8_encode('XXXXXXXXXX'), $b64)),
        'Cache-Control: no-cache'
    ];
$ch = curl_init('https://api.sandbox.gemini.com/v1/balances');
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
echo curl_exec($ch);

Error:

{"result":"error","reason":"InvalidSignature","message":"InvalidSignature"}

>Solution :

I don’t have any access to the API, but could you please try the following chagnes:

1. Remove the MD5 function from the signature calculation. The hash_hmac function in PHP already does this by default.

'X-GEMINI-SIGNATURE: ' . hash_hmac('sha384', utf8_encode('XXXXXXXXXX'), $b64),

2. Switch around the payload and the key in the same line:

'X-GEMINI-SIGNATURE: ' . hash_hmac('sha384', $b64, utf8_encode('XXXXXXXXXX')),