Hmmm... but wouldn't it immediately change again as soon as the callback exits, anyway, as the foreach goes into its next iteration? I was thinking more along the lines of whether the order of either of the two arrays is (or is supposed to be) significant, whether said order might be changed by any of the callbacks, whether the callbacks are (or are supposed to be) idempotent, and so on -- all of which requires knowing what the arrays contain or are supposed to contain.