php-judy is an exentsion by Nicolas Brousse for the Judy C library.
http://github.com/orieg/php-judy
A Judy array is a complex but very fast associative array data structure for
storing and looking up values using integer or string keys. Unlike normal
arrays, Judy arrays may be sparse; that is, they may have large ranges of
unassigned indices.
http://en.wikipedia.org/wiki/Judy_array
Judy is a C library that implements a dynamic array. Empty Judy arrays are
declared with null pointers. A Judy array consumes memory only when
populated yet can grow to take advantage of all available memory. Judy's key
benefits are: scalability, performance, memory efficiency, and ease of use.
Judy arrays are designed to grow without tuning into the peta-element range,
scaling near O(log-base-256) -- 1 more RAM access at 256 X population.
http://judy.sourceforge.net