This change does not break BC. The "a" format in pack/unpack functions has not changed at all in 5.5. It still does the same thing and it was not replaced by the new "Z" format option, which I added. They do different things.
The "a" format strips null bytes from the string. The new "Z" format in 5.5 truncates the string at the first null-byte, returning everything prior to it. pack/unpack functions should still be backwards-compatible with PHP < 5.5.
