First a side note - any of this does not matter in a real production application, as the time differences are superficial and the optimization of the application should be done in other places (dealing with network, database, filesystem, etc.). That being said, for curiosity's sake:
implode
may be more efficient concatenation, but only if you already have the array. If you don't, it probably would be slower since all the gain would be offset by the time needed to create the array and allocate its elements. So keep it simple :)
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