Setting Up OPCache to Improve Performance

Introduction to OPCache

OPCache is a built-in PHP extension that improves performance by storing precompiled script bytecode in shared memory, thus eliminating the overhead of parsing and compiling PHP scripts on each request. Originally introduced in PHP 5.5, it has become an essential tool for accelerating PHP applications in production environments.

What Is OPCache

OPCache intercepts the PHP compilation process, caches the compiled opcode, and reuses it for subsequent requests. By preventing redundant parsing and compilation, it reduces CPU usage and response time. For official details, see the PHP manual:
https://www.php.net/manual/en/book.opcache.php.

Key Benefits of OPCache

• Faster Script Execution – Bytecode reuse skips parsing/compiling overhead.
• Reduced CPU Load – Less time spent in the Zend engine.
• Scalability – Handles higher concurrency with the same hardware.
• Memory Efficiency – Shared memory store for all PHP processes.

Prerequisites and Compatibility

• PHP 5.5 or later (bundled by default).
• Access to php.ini or conf.d/opcache.ini.
• Sufficient shared memory (128MB or more recommended).
• Note: OPCache is not a userland cache it does not store application data.

Installing and Enabling OPCache

Step 1: Verify PHP Version

php -v

If PHP gt= 5.5, OPCache is bundled. Otherwise, install it via PECL:

Step 2: Install via PECL (if needed)

pecl install zendopcache

Add the extension to php.ini:

zend_extension=opcache.so

Step 3: Restart Your Web Server

service php-fpm restart
service apache2 restart
service nginx restart

OPCache Configuration Directives

All OPCache settings reside in php.ini or a dedicated opcache.ini. Below is a table summarizing key directives:

Directive	Default	Description
opcache.enable	1	Enable OPCache for web requests.
opcache.memory_consumption	128	Shared memory size in MB.
opcache.interned_strings_buffer	8	Memory for interned strings.
opcache.max_accelerated_files	10000	Max number of cached scripts.
opcache.validate_timestamps	1	Revalidate file timestamps.
opcache.revalidate_freq	2	How often (sec) to check file changes.

Recommended Base Configuration

opcache.enable=1
opcache.memory_consumption=256
opcache.interned_strings_buffer=16
opcache.max_accelerated_files=20000
opcache.validate_timestamps=1
opcache.revalidate_freq=2
opcache.save_comments=1
opcache.fast_shutdown=1
  

Integrating OPCache with PHP-FPM CLI

• PHP-FPM: Ensure each pool uses the same opcache.ini or include directives in the global php-fpm.conf.
• CLI: OPCache is disabled by default in CLI mode. To enable:

  php -dopcache.enable_cli=1 yourscript.php

Monitoring and Maintenance

• Regularly review memory usage to avoid fragmentation.
• Adjust max_accelerated_files if your application loads more files.
• Plan cache resets during maintenance windows: opcache_reset().

Troubleshooting Common Issues

• Scripts Not Updating: Increase revalidate_freq or disable validate_timestamps in stable environments.
• Out of Shared Memory: Raise memory_consumption or restart PHP-FPM to clear fragmentation.
• Low Hit Rate: Ensure opcache.enable is set to 1 and monitor with status pages.

Advanced Techniques

• Preloading (PHP 7.4 ): Load critical classes/functions at server start via opcache.preload. See
  Preloading Documentation.
• JIT Compiler (PHP 8 ): Enable OPCaches JIT settings for computationally intensive workloads.

Security Considerations

• Keep validate_timestamps enabled in development to prevent stale code execution.
• Restrict access to OPCache status pages via IP whitelisting.
• Avoid caching user-uploaded code or dynamic eval’ed scripts.

Conclusion

Properly configured OPCache can dramatically improve PHP application performance, lower server load, and deliver a snappier user experience. By following best practices—allocating sufficient memory, tuning file validation, and leveraging monitoring tools—you’ll ensure your applications run efficiently at scale.