Arrays are at the core of PHP and act as the swiss army knife for data structures. They are known as an efficient and generic solution. But this is only half of the truth. The internal C representation as a hash table causes a non-intuitive performance behaviour of arrays. Associative arrays for example scale badly in key size and the different accessing functions like array_pop() and array_shift() behave very differently in respect to performance. I made a quick benchmark to visualize the difference. At least in loops it is more efficient to reverse the array and pop it instead of simply shifting it. This is because an array_shift() - strange enough - forces a reindexing operation for each shift.