[1] Total Execution Time find_uniq: 5.8177680969238
[2] Total Execution Time find_uniq: 28.545898914337

Where [1] is my solution vs [2] this solution, iterated over 100000000 array elements

All numbers are equal except for one.

Now it doesn't (sometimes, depending on selected PHP's version), because performance test were added later.

Please use spoiler flag next time, and read this:

All numbers are equal except for one.

This code does not work

That will also fail on decimals, I known because it was my first try as well :')

I see. I misunderstood the kata.

All numbers are equal except for one.

The test isn't comprehensive. This solution will fail this test:

$this->assertEquals(2, find_uniq([1, 1, 1, 2, 1, 1, 3, 3]));