PHP 8.0.12:
  
  $ time php fib.php 42
  42 433494437
  
  real 0m29.274s
  user 0m29.003s
  sys 0m0.105s

  $ time php -dopcache.enable_cli=1 fib.php 42
  42 433494437

  real 0m26.341s
  user 0m26.179s
  sys 0m0.058s
  ----------------------------------------
  PHP 7.3.5:
  $ time php fib.php 42
  42 433494437

  real 10m23.573s
  user 10m16.592s
  sys 0m1.828s
  ----------------------------------------
  Settings for PHP 8:
  PHP Version => 8.0.12
  auto_globals_jit => On => On
  pcre.jit => 1 => 1
  opcache.jit => tracing => tracing
  opcache.jit_bisect_limit => 0 => 0
  opcache.jit_blacklist_root_trace => 16 => 16
  opcache.jit_blacklist_side_trace => 8 => 8
  opcache.jit_buffer_size => 0 => 0
  opcache.jit_debug => 0 => 0
  opcache.jit_hot_func => 127 => 127
  opcache.jit_hot_loop => 64 => 64
  opcache.jit_hot_return => 8 => 8
  opcache.jit_hot_side_exit => 8 => 8
  opcache.jit_max_exit_counters => 8192 => 8192
  opcache.jit_max_loop_unrolls => 8 => 8
  opcache.jit_max_polymorphic_calls => 2 => 2
  opcache.jit_max_recursive_calls => 2 => 2
  opcache.jit_max_recursive_returns => 2 => 2
  opcache.jit_max_root_traces => 1024 => 1024
  opcache.jit_max_side_traces => 128 => 128
  opcache.jit_prof_threshold => 0.005 => 0.005