In this recent blog post nikic takes an in-depth look at how large PHP arrays really are - how memory is used in the creation and management of these handy PHP variable types.

In this post I want to investigate the memory usage of PHP arrays (and values in general) using the following script as an example, which creates 100000 unique integer array elements and measures the resulting memory usage. [...] How much would you expect it to be? [...] Now try and run the above code. You can do it online if you want. This gives me 14649024 bytes. Yes, you heard right, that's 13.97 MB - eightteen times more than we estimated.

He goes into the details of PHP's memory management and breaks it down into the different totals (for 64 bit and 32 bit OSes) and details on each - zvalue_value, zvalue, cycles collector, Zend MM allocator and the buckets used to isolate one array (hash table/dictionary) from another.

What does this tell us? PHP ain't C. That's all this should tell us. You can't expect that a super dynamic language like PHP has the same highly efficient memory usage that C has. You just can't.