This is one of those “I know this is possible, but don’t know how”.
php -r '$parts = [ "hello", "world" ]; echo implode( " ", $parts ) . PHP_EOL;' # hello world
Only useful for simple onelines. You’re very likely to be better of putting code in a php file and run that script like:
Run PHP with WordPress loaded.
Of course your good old friend wp-cli can help.
It can run code with wp fully loaded. So if you add things in the init action or even after the wp_loaded, those plugin/theme functions, posttypes and such are all available.
First off the plain php code execution, with wp eval
wp eval 'var_dump(did_action( "wp_loaded" ));' # int(1)
Secondly we execute a file with WordPress fully loaded using wp eval-file
wp eval-file ./helloworld.php