Use a custom php.ini with mod_php or php as a cgi

Describes in exhaustive detail how to change configuration settings and implement a custom php.ini file for use with the Apache Web Server.

PHP run as Apache Module (mod_php)

Add this to your root .htaccess file. Say your php.ini is in folder /askapache.com/ini/

SetEnv PHPRC /askapache.com/ini