Seems that you are working with base64; in php you are right pack and unpack is your friends.
example
in Node
$ node
> Buffer('hello world').toString('base64')
aGVsbG8gd29sZA==
in PHP
$ php -a
php > echo base64_encode('hello world');
aGVsbG8gd29ybGQ=
But if you are only looking for the binary:
in Node
> Buffer('hello wold')
<Buffer 68 65 6c 6c 6f 20 77 6f 6c 64>
in PHP
php > print_r(unpack('H*', 'hello world'));
Array
(
[1] => 68656c6c6f20776f726c64
)
So in your instance you would first decode the base64 and then unpack it.
php > $raw = base64_decode('GHlDHToiZA1ViUu+W+EXww==');
php > print_r(unpack('H*', $raw));
Array
(
[1] => 1879431d3a22640d55894bbe5be117c3
)
Easy peasy ;)