In recent post to his blog Giorgio Sironi takes a look at a handy feature that PHP makes it relatively simple to work with objects in your applications - lazy loading.

What is lazy loading? A practice in object/relational mapping which simulate the presence of the whole object graph in memory. There is some techniques for produce this illusion that I explain here. [...] The main problem is that we cannot load all the object graph, because it will not fit in the memory of the server and it will take much time to build, depending on the size of the database. Nor we can load an arbitrary subset of it.

His solution is to use the proxy design pattern to create an interface allowing only the first level of information to be loaded by default. After that, each of the sub-levels have what they need to load themselves and the data that might be needed inside.