PHP.ini mode constants.

I have a aways thought that I could just use php in the wp-content/themes/themesname/functions.php and just add a line of ini_set() and change them however I like.

But, then when I see some of my code didn’t work, for some reason there is a note on php.net and says that there is so called INI mode constants

I kinda find this annoying, once again I need to change my php.ini and my .htaccess.