As xil3 wrote, most people do look at a reference (that's what they're for) when they code. I personally have the php.net search tool installed on Firefox. I also have a search keyword set up for it. So I can just type php array or php array_slice into the address bar and it will take me to the Arrays page and the documentation for array_slice() automatically. Aptana also gives you tooltips about each function, but they're fairly brief, out of necessity.
Generally, the functions you use regularly you will remember how to use. The ones you don't or haven't used in a while you'll just have to look up. However, as a general rule, most of the array functions that are for modifying arrays are usually passed by ref, e.g.:
array_pop()
array_push()
array_shift()
array_unshift()
array_walk()
asort()
array_multisort()
Whereas the ones that make computations using multiple arrays or are extracting something from the array generally are passed by value, e.g.:
// multiple array inputs
array_diff()
array_merge()
array_combine()
// extraction
array_values()
array_keys()
array_unique()
array_sum()
Of course, there are some that break the rules or have ambiguous names, like array_reverse(), and I often still get array_map() and array_walk() confused (the latter is by ref), but for the most part it becomes intuitive after a while.
Edit:
The PHP.net search tool can be found on MyCroft. I use the one labeled "PHP Function List - en" by Lucas.