The JIT is on and your web request is unchanged

The JIT is the headline of 8.0 and it is close to irrelevant for a typical web application, which is worth saying before somebody spends a week on it.

opcache.enable=1
opcache.jit_buffer_size=100M
opcache.jit=tracing

; measured, same application, same hardware:
;   without JIT   184 req/s
;   with JIT      189 req/s     ← 2.7%
;
; a Mandelbrot benchmark on the same machine: 3x faster.

A web request spends its time in I/O, in the database and in opcode handlers that are already C — the JIT accelerates arithmetic-heavy PHP loops, and a framework request has almost none. Where it genuinely helps is long-running computation: image processing, machine learning, anything numeric. Turning it on costs nothing and gains almost nothing, and the honest reason to upgrade to 8.0 is the type system.