Opcache - How PHP Compiles & Caches Bytecode

# ── How PHP executes a request WITHOUT Opcache ─────────────────
# 1. PHP reads raw source files (fopen + fread)
# 2. Lexer tokenises source into tokens
# 3. Parser builds Abstract Syntax Tree (AST) from tokens
# 4. Compiler converts AST to Zend opcodes (bytecode)
# 5. Zend Engine executes opcodes
# Steps 1-4 happen on EVERY request - even if files haven't changed!
#
# ── WITH Opcache ───────────────────────────────────────────────
# First request:    Steps 1-4 run, result stored in shared memory
# All later requests: Skip to step 5 - execute from cached bytecode
# Result: ~10x faster startup

# ── php.ini Opcache configuration (production settings) ────────
[opcache]
opcache.enable                 = 1
opcache.enable_cli             = 0          ; Don't cache CLI runs (dev loops)
opcache.memory_consumption     = 256        ; MB of shared memory
opcache.interned_strings_buffer = 16        ; MB for interned strings pool
opcache.max_accelerated_files  = 20000      ; Max files in cache (>total PHP files in project)
opcache.validate_timestamps    = 0          ; PRODUCTION: don't check mtime (use 1 in dev)
opcache.revalidate_freq        = 0          ; Seconds between mtime checks (irrelevant if validate=0)
opcache.fast_shutdown          = 1          ; Faster request shutdown
opcache.save_comments          = 1          ; Keep docblocks (needed for Doctrine/annotations)
opcache.preload                = /var/www/html/preload.php  ; PHP 7.4+: preload important classes

# ── validate_timestamps = 0 in production ──────────────────────
# With validate_timestamps=0: PHP never checks if source files changed.
# After deploying new code: must call opcache_reset() or restart PHP-FPM!
# Without restart: old bytecode still served - users see old code!

# ── preload.php - PHP 7.4+ preloading ────────────────────────
# Classes preloaded into shared memory at PHP-FPM startup
# Available to ALL workers without re-compiling
# Only for classes that are used on EVERY request

<?php
// preload.php - preload core framework classes
$classes = [
    __DIR__ . "/src/Http/Request.php",
    __DIR__ . "/src/Http/Response.php",
    __DIR__ . "/src/Http/Router.php",
    __DIR__ . "/src/Blog/Domain/Post.php",
    __DIR__ . "/src/Blog/Service/PostService.php",
];
foreach ($classes as $file) {
    opcache_compile_file($file);
}

<?php
// ── Inspecting Opcache at runtime ─────────────────────────────
$status = opcache_get_status();
echo "Cache full: "           . ($status["cache_full"] ? "YES - increase memory!" : "No") . "
";
echo "Cached scripts: "       . $status["opcache_statistics"]["num_cached_scripts"] . "
";
echo "Cache hits: "           . $status["opcache_statistics"]["hits"] . "
";
echo "Cache misses: "         . $status["opcache_statistics"]["misses"] . "
";
echo "Hit rate: "             . round($status["opcache_statistics"]["opcache_hit_rate"], 2) . "%
";
echo "Memory used: "          . round($status["memory_usage"]["used_memory"] / 1048576, 2) . " MB
";
echo "Memory free: "          . round($status["memory_usage"]["free_memory"] / 1048576, 2) . " MB
";

// Invalidate one file after deploy
opcache_invalidate("/var/www/html/src/Blog/Service/PostService.php", force: true);

// Clear entire cache (call after deployment)
opcache_reset();