All the PHP modules which have ini dierectves associated with them will be displayed inthe following way in the PHP documentation, which will give you information on when and where they can be set. Some directives can be set only in the php.ini file, PHP_INI_SYSTEM and some can be set anywhere, PHP_INI_ALL.

directive.name "default" PHP_INI_*

For settings which are PHP_INI_ALL or PHP_INI_USER, you can use the ini_set() function, tochnage the values of in your script. These changes are however volatile and the settings will go back to their original as per defualt or php.ini file the next time the script is called.