Opcode Optimisation

Also Known As

TL;DR

Explanation

PHP's Optimizer (part of OPcache) applies multiple optimisation passes to compiled opcodes: constant folding (2 + 3 compiled to 5), dead code elimination (if (false) { } removed), array/string merging (concatenated literals merged), pass-through optimisation (unnecessary RETURN opcodes removed), and live variable analysis (unused variables eliminated). PHP 7+ uses type inference during optimisation, enabling more aggressive dead code elimination with typed code. These optimisations happen once at compile time and are cached — they have zero runtime overhead once in OPcache.

Common Misconception

Why It Matters

Common Mistakes

• Pre-computing constants manually — PHP optimiser does this at compile time.
• Avoiding clear code for micro-optimisation — the optimiser handles constant expressions.
• Not enabling OPcache — without it, no optimisations apply on any request.
• Thinking opcache.optimization_level controls all passes — different passes have different levels.

Code Examples

// Unnecessary manual micro-optimisation:
$secsPerDay    = 86400;          // Pre-computed manually
$secsPerWeek   = 604800;         // Clutter — compiler does this
$bytesPerMB    = 1048576;

// Complicated null check to avoid function call:
if (isset($arr['key']) && $arr['key'] !== null) { }  // Premature

// Let the compiler optimise:
$secsPerDay  = 60 * 60 * 24;     // Compiler folds to 86400
$secsPerWeek = 60 * 60 * 24 * 7; // Compiler folds to 604800
$bytesPerMB  = 1024 * 1024;       // Compiler folds to 1048576

// Readable null check:
if (isset($arr['key'])) { }  // Clear intent, same performance

// View opcache stats:
$status = opcache_get_status();
// Check: ['opcache_statistics']['hits'] vs ['misses']
// High miss rate = too many files or opcache.max_accelerated_files too low

References

• ↗ https://www.php.net/manual/en/opcache.configuration.php

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