APC (as in http://us3.php.net/manual/en/book.apc.php) stands for Alternative PHP Cache. What is it alternative to - is there another, built-in caching framework in PHP?
Memcached is also widespread.
Also see this List of PHP accelerators which speak of:
- Alternative PHP Cache
- eAccelerator
- ionCube PHP Accelerator
- Turck MMCache
- XCache
- Nusphere PhpExpress
- Zend Accelerator
- Zend Platform
- Windows Cache Extension for PHP
There is no built-in OP-Code or variable cache built in. But there are a number of alternatives...
I believe it was an alternative to the proprietary (and commercial) Zend Cache which was around in the late '90s and early 2000's. There are a number of different accelerators out there now (each with their own advantages and disadvantages), but in the early 2000's, the main one was Zend Cache. So I would assume (the time scales look right) that APC was meant to be an alternative to Zend Cache (an open source and free alternative)...
An alternative to not using one at all?
Seriously though, maybe as an alternative to eAccelerator and Xcache?
Also see: http://stackoverflow.com/questions/930877/apc-vs-eaccelerator-vs-xcache