To reinforce a point he's made before (references in PHP are bad) Johannes Schluter has posted an example to his blog of a specific instance that causes an (expected) issue with references and foreach loops.

Now there is one use case which leads to an, at first, unexpected behavior which I didn't see as a real live issue when I stumbled over it at first, but then there were a few bug reports about it and recently a friend asked me about it ... so here it goes.

He show a code snippet of looping over an array with two foreaches and a print_r that shows the bug - the array changed from the original for no clearly apparent reason. To understand why this happens, he goes into detail on how variables are handled - complete with graphs.