unpack() - how to handle bitfields??

Hi all, I'm wrestling with this problem for a while now. I want to read a binary file (created by C software) using PHP. I know of unpack(). But the C struct that is stored in the binary file contains variables with bitfields: struct some_struct { char val_a : 2 int val_b : 7 } Such a construct means that val_a is char with a bitfield of 2 and therefore can contain values between 0 (00 binary) and 3 (11 binary). val_b is an integer with a range of 0 (0000000) to 127 (1111111). Now the problem is: how on earth do I handle those values with PHP's unpack()?!? I've tried a lot of syntaxes but none of them seem to work. Can someone please shortcut whether it is possible or not and if yes, *how*? Thanks in advance, Tim