PHP Opcache Preloading Explained

OPcache preloading loads selected classes/functions during PHP-FPM startup so they persist for its lifetime.

What It Does and When to Use It

Count preloaded script paths from the active OPcache status; status may be unavailable in the CLI SAPI. OPcache caches compiled opcodes, JIT may optimize CPU-intensive PHP, while APCu caches application-owned values.

Requirements, Inputs and Expected Behavior

Confirm the runtime, extension, environment variables, and permissions before running the examples in production. Preloaded code changes require process restart; preloading is not the same as ordinary opcode caching.

Example 1: Basic Operation

Count preloaded script paths from the active OPcache status; status may be unavailable in the CLI SAPI.

<?php
$status=opcache_get_status(false);
$preloaded=$status['preload_statistics']['scripts'] ?? [];
echo 'Preloaded scripts: ',is_array($preloaded)?count($preloaded):0;

Example 2: Behavior and Validation

The generated preload.php must refer to actual files and be configured via opcache.preload; writing it alone does not preload anything.

<?php
$preload = <<<'PHP'
<?php
require __DIR__ . '/src/Domain/Order.php';
opcache_compile_file(__DIR__ . '/src/Domain/Invoice.php');
PHP;
file_put_contents(__DIR__ . '/preload.php', $preload);

Practical Example: Application Use

Configure FPM preloading with an executable script; changes require restarting FPM workers.

opcache.enable=1
opcache.preload=/srv/app/preload.php
opcache.preload_user=www-data

Important Behavior and Edge Cases

The second example highlights: The generated preload.php must refer to actual files and be configured via opcache.preload; writing it alone does not preload anything. The practical example adds: Configure FPM preloading with an executable script; changes require restarting FPM workers.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• OPcache preloading loads selected classes/functions during PHP-FPM startup so they persist for its lifetime.
• Configure FPM preloading with an executable script; changes require restarting FPM workers.
• Profile application response times and memory under representative traffic before tuning.