This PHP class is used for economical storage of large vectors of 0/1 values. Bit vectors are useful, for example, in prime number generation. The native PHP array is much too powerful for the job, and far too costly in memory: the overhead is 208 bytes per element (on a 64-bit system), or 1667 times the optimum size. Class bitvec stores packed bits in an SplFixedArray of integers, considerably reducing the storage required.