Different results in hmac encoding between Dart and PhP - php

Question

I'm trying to encode a message in flutter and to verify it with some php code, but somehow there are some differences in the hmac encoding. The header and payload values are the same in both cases. But somehow there are some differences between the resulting values.
Any help would be very helpful, I'm stuck on this for some days now.
$base64UrlHeader = 'header';
$base64UrlPayload = 'payload';
$secret = 'secret';
$signature = hash_hmac('sha256', $base64UrlHeader . "." . $base64UrlPayload, $secret, true); // 9b491a7aa29955d9d67e302965665ba0cfa4306c00470f8946eb6aa67f676595
$base64UrlSignature = base64UrlEncode($signature); // sYql52zk6tqYeGSUsDv_219UtgpK3c8-TMuko4n_L5Q
function base64UrlEncode($text) {
return str_replace(
['+', '/', '='],
['-', '_', ''],
base64_encode($text)
);
}
And this is my dart code:
// This values are obtained by using the _base64UrlEncode method from below,
// I just wrote the values directly here not to clutter with code
var base64UrlHeader = 'header';
var base64UrlPayload = 'payload';
/// SIGNATURE
var secret = utf8.encode('secret');
var message = utf8.encode(base64UrlHeader + '.' + base64UrlPayload);
var hmac = new Hmac(sha256, secret);
var digest = hmac.convert(message); // b18aa5e76ce4eada98786494b03bffdb5f54b60a4addcf3e4ccba4a389ff2f94
var signature = _base64UrlEncode(digest.toString()) // YjE4YWE1ZTc2Y2U0ZWFkYTk4Nzg2NDk0YjAzYmZmZGI1ZjU0YjYwYTRhZGRjZjNlNGNjYmE0YTM4OWZmMmY5NA
// This method is used to obtain the header, payload and signature
static String _base64UrlEncode(String text) {
Codec<String, String> stringToBase64Url = utf8.fuse(base64url);
return stringToBase64Url
.encode(text)
.replaceAll('=', '')
.replaceAll('+', '-')
.replaceAll('/', '_');
}
Both the header and payload are obtained from the same json object,