unpack on offsets in binary data in variable?

Hi, I'm searching for an efficient way to use unpack on binary data within a variable (php 5.1.something). In the past I was doing fopen fseek unpack( fread(...) fseek etc. Now this time I need to do the same, but I don't have an actual file: my data is only in a variable and I don't want to operate on a file but the variable within. I was thinking about using substr() and passing the small chunks of data I need to the unpack() function. Question: does this work without problems on arbitrary binary, maybe containing zero-byte values reliable? thanks, - Markus