opcache.file_cache_read_only examples

Examples of pre-warming the opcache on PHP 8.5, for:

• Linux based containers, i.e. Docker
• Application code is read-only after container build

These examples are primarily for Google Cloud's App Engine, which builds Docker containers using buildpacks, which include all application code that remains read-only at runtime.

Examples

• Basic example with single public/index.php file.
• Intermediate example using Symfony's Finder to compile all project files.

Notes

The opcache file based cache is only valid for the same build of PHP (not just a specific version match), so it's important that the opcache is generated at deploy / container build time with the exact same build of PHP.

This is due to zend_system_id being included in both the file path:

https://github.com/php/php-src/blob/85f7e5477a5cd81c221c3cc451bc76bdf3c8ac83/ext/opcache/zend_file_cache.c#L981-L1006

And also, in the cache files themselves:

https://github.com/php/php-src/blob/85f7e5477a5cd81c221c3cc451bc76bdf3c8ac83/ext/opcache/zend_file_cache.c#L954-961

You can debug this by setting opcache.log_verbosity_level=4.

On startup, if the file cache is enabled, you'll see a message with the PHP build ID:

https://github.com/php/php-src/blob/85f7e5477a5cd81c221c3cc451bc76bdf3c8ac83/ext/opcache/ZendAccelerator.c#L3332-L3335