PHP’s JIT (added in 8.0) compiles hot code paths to native machine code. For typical web requests the win is small — the request cycle does not run any single function long enough.
The clear wins are CPU-bound workloads: image processing, encryption, JSON encoding of large structures, and mathematical operations. Benchmarks in those areas show 2–3× speedups.
Enable it with opcache.jit_buffer_size=128M and opcache.jit=tracing. Profile before and after; the JIT can occasionally pessimise specific code paths.