In a new article to his blog, Ryan Campbell has expressed some concerns in the amounts of memory that some of the object oriented practices in PHP are using these days.
It's hard to imagine pushing the limits of object oriented PHP so far that your web servers choke, but the truth is those limits are reached faster than you think. [...] While replacing objects with arrays when possible makes things a little better, the most performance friendly approach involves appending strings. For your convenience, we've run some tests that measure page execution times and memory usage to create the following guideline to help you plan out what areas of your code may have to break away from an object oriented nature.
He shares some benchmarks of the memory use for different variable types - strings, arrays and objects - giving load times and the amounts of memory used. He mentions three workarounds that could help (unset, static methods and paging) reduce the memory consumption of your script.