What does memory_get_usage() actually do ? Does it report the "heap" size assigned to the process, or does it use PHP internal counters for the allocated user data/variables ? A C malloc subsystem will assign a whole lot of virtual memory, in steps of pages, or more if it decides to attach a piece using mmap().

In order to make this test case relevant, I'd say one have to know what memory_get_usage() does - it's at least meaningless to determine the overhead of an array based on it, if for whatever reason creating the 1. PHP array in a program also initializes "big" memory pools that count towards the memory usage.