Some people may say: My job is creating forms in WordPress, why would I learn things that have no direct connection to my job and that I won’t ever need in real life. And my answer is that if you don’t learn them, then making those WordPress forms will be your job for your entire life.
General programming awareness
Understanding the “big O notation”, what it means and why it matters
Array data structures as they are defined, not as they are implemented in PHP. How is the n-th element of an array accessed in memory.
At least one other fundamental data structure (linked list, queue, stack)
Being able to implement at least one O(NlogN) sorting algorithm
Understanding recursion. This really goes without saying