Essential Array Functions
count() // number of elements
array_push() // add to end
array_pop() // remove from end
array_shift() // remove from start
array_unshift() // add to start
in_array() // check if value exists
array_key_exists() / isset()
array_keys() // get all keys
array_values() // get all values
array_merge() // merge arrays
array_diff() // difference
array_intersect() // intersection
array_reverse() // reverse
array_unique() // remove duplicates
array_slice() // extract portion
array_chunk() // split into chunks
array_map() // apply callback
array_filter() // filter with callback
array_reduce() // reduce to single value
array_search() // find key by value
array_flip() // swap keys and values
array_combine() // keys from one, values from another