Can anyone explain how this usort actually works

Well, usort is only useful when you do not care for key association, and you need a special, complex(ish) logic to sort, like in your example, where you are sorting based on a specific key. If you have just a normal [3,5,6,4,2,1,9,8] array, then "asort" will probably be enough. Take a looksy for all the available options: http://php.net/manual/en/array.sorting.php

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