When building API-like applications in PHP, one issue will always remain; a high load will eventually slow down everything. To prevent over-loading, anyone will come to the conclusion a cache is needed. The most access calls should be served without recalculating anything to free up CPU-time.
Caching can be done on Disk, in a database (indirectly also Disk-caching), or directly in memory.
To setup MemCache and use this memory-caching-system, read on :)