There is probably a little overhead, but it's nothing I've ever worried about; but if you ever get the sort of traffic load where it would actually matter, you can probably afford a 2nd server along with a load-balancer. (Hardware is cheap these days compared to paying programmers for weeks/months of debugging code that is poorly designed/written. )
For that matter, PHP arrays are somewhat notorious for using excessive memory (at least in some people's view), so you may still be ahead of the curve by creating objects custom-designed for the specific data you want to store/manipulate -- again pointing to the fact that fiddling around with micro-optimizations can be tricky and sometimes counter-intuitive. (If you really need to worry about every micro-second for a missile guidance system or such, you probably shouldn't be using PHP anyway. )
Ultimately, I'm generally in favor of designing/coding for maintainability, and worrying about optimization only if/when you find you need it (though you always have it sort of kicking around in the back of your brain to make sure you aren't doing anything really stupid).