A fast database for constant data with memory sharing across processes

CHDB (constant hash database) is a fast key-value database for constant data,
realized by using a memory-mapped file and thus providing the following
functionalities:
- Extremely fast initial load, regardless of the size of the database.
- Only the pages of the file which are actually used are loaded from the disk.
- Once a page is loaded it is shared across multiple processes.
- Loaded pages are cached across multiple requests and even process recycling.
A typical use of CHDB is as a faster alternative to defining many PHP
constants.
CHDB is internally implemented as a hash-table using a perfect hashing function,
thus guaranteeing worst case O(1) lookup time.