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They ARE implemented as COW. It's discouraged to use explicit reference passing if you don't want to alter the array as PHP has to do a slight bit more work if you pass a reference.

I think copy on write was added in the 5.1 timeframe (around 2005)

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Thanks for the info. I do think the default should have been pass by reference though, like with objects since PHP5.

Arrays (and everything else) should be objects. That would also solve the pass-by-reference issue and would clean up the global namespace that's littered with array_* functions.

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