associative arrays will be fast but will take up more memory space.function and class is almost smae but cl;ass has an advantage that it can be resuable and easy to maintain and looks more professional

What makes you think they take up more memory space?

how much memory space an array takes depends on the way you use it.
But i have generally seen people using it inefficiently i.e. they just fetch whole dataset in array when they just need few data fields.
Also i have seen that many people when they use array they tend to forget to free up memory space which array took,
However if you are making a simple script or problem memory considerations are not so significant.