You can do this with normal file methods using the data:// protocol:
$data = file('data://text/plain;charset=utf-8;base64,dGhpczENCnRoaXMyDQp0aGlzMw0KdGhpczQ=');
Alternatively though you can pull out the base64 encoded string and decode it yourself:
$plaintext = base64_decode('dGhpczENCnRoaXMyDQp0aGlzMw0KdGhpczQ=');