List of Big-O for PHP functions

After using PHP for a while now, I've noticed that not all PHP built in functions as fast as expected. Consider the below two possible implementations of a function that finds if a number is prime using a cached array of primes.

array_unique vs array_flip

If I had an array of signed integers e.g:

Array

(

    [0] => -3

    [1] => 1

    [2] => 2

    [3] => 3

    [4] => 3

)

Why isn"t LinkedList.Clear() O(1)

I was assuming LinkedList.Clear() was O(1) on a project I'm working on, as I used a LinkedList to drain a BlockingQueue in my consumer that needs high throughput, clearing and reusing the LinkedList afterwards.