How to detect if PHP JIT is enabled

php -i | grep "opcache"

checking:
opcache.enable => On => On
opcache.enable_cli => On => On
opcache.jit => tracing => tracing

opcache_get_status() will not work when JIT is disabled and will throw Fatal error.

echo (function_exists('opcache_get_status') 
      && opcache_get_status()['jit']['enabled']) ? 'JIT enabled' : 'JIT disabled';

We must have following settings for JIT in php.ini file.

zend_extension=opcache.so
opcache.enable=1
opcache.enable_cli=1 //optional, for CLI interpreter
opcache.jit_buffer_size=32M //default is 0, with value 0 JIT doesn't work
opcache.jit=1235 //optional, default is "tracing" which is equal to 1254

You can query the opcache settings directly by calling opcache_get_status():

opcache_get_status()["jit"]["enabled"];

or perform a lookup in the php.ini:

ini_get("opcache.jit")

which is an integer (returned as string) where the last digit states the status of the JIT:

0 - don't JIT
1 - minimal JIT (call standard VM handlers)
2 - selective VM handler inlining
3 - optimized JIT based on static type inference of individual function
4 - optimized JIT based on static type inference and call tree
5 - optimized JIT based on static type inference and inner procedure analyses

Source: https://wiki.php.net/rfc/jit#phpini_defaults