The Judy family of functions supports fully dynamic arrays. These arrays may be indexed by a 32- or 64-bit word (depending on the processor) or a string. Judy arrays are both speed- and memory-efficient, with no tuning or configuration required, across a wide range of index set types (sequential, periodic, clustered, random).
As far as I know, there no PHP [2]implementation of Judy yet.
[1] http://www.sourcejudy.com
[2] http://php.net/array
