An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices.
Write a function that, given a sequence, returns its equilibrium indices (if any). Assume that the sequence may be very long. Feel free to Google for hints, but give lots of comments about what your thoughts are and your process for devising an algorithm.
phpunit EquilibriumIndexTest.php