Wouldn't it be even greater if you try to read the detailled docs and tell us what you don't understand? ;-)

Your hash really looks like a perverted array ...

try to figure out how many lookups are performed and if they can be grouped in smaller data structures.

BTW: If your university prefers PHP but accepts blocking large parts of the RAM (6 million hash entries can easily result in 1GB or more memory consumption) something seems terribly wrong...