PHP Preloading is a mechanism that enables any application to /warm-up/ a collection of files by pre-compiling them, and keeping the generated byte code in memory.

OPcache can only be compiled as a shared extension. If you have disabled the building of default extensions with --disable-all, you must compile PHP with the --enable-opcache option for OPcache to be available. Once compiled, you can use the zend_extension configuration directive to load the OPcache extension into PHP.

Preloading is built on top of OPcache, and it does what it says on the tin - it preloades specific PHP files or classes into memory before they are needed. Having these files already available when the script runs saves time and resources because PHP doesn't have to load them again for each request.

OPcache is a type of caching system that saves precompiled script bytecode in a server's memory called a cache, so each time a user visits a web page, it loads faster.