优雅的排序方式，独特并消除数组中的重复项

Question

Of course I can sort an array sort(), eliminate duplicates array_unique() and eliminate blanks array_filter(). I could do that in three lines, and then repeat those three lines for the ten arrays I have to process.

But I wanted it to be at least slightly elegant, so I tried to combine all three operations. It did work for the first two, then I pushed it too far and applied the sort()

$testArray = sort(array_filter(array_unique($testArray)));

This produced:

Strict Standards: Only variables should be passed by reference

So what would be the most elegant way to accomplish this array processing goal?

Bonus points for helping me understand why it failed.

Answer 1

Just as a prove of concept you can kind of avoid creating in-between variables and mutating the original array. Take a look at SplMinHeap from Standard PHP Library (SPL). You can use this class for immutable sorting:

$testArray = iterator_to_array(array_reduce(
    array_filter(array_unique($testArray)),
    function ($heap, $element) {
        $heap->insert($element);
        return $heap;
    },
    new SplMinHeap
));

Here is working demo.