The php.ini file is a file used to configure PHP. Note that there may be several php.ini files on your computer. For example, it is likely that there is a php.ini file in your Apache installation, which specifies the configuration of PHP when it is called by Apache. There may be another php.ini file which is used when you are calling PHP from the command line.
If you don't know which php.ini file is being used in a particular circumstance, you can find out by invoking the following command:
php -i | grep php.ini
You can also visit the tiki-phpinfo.php file of your tiki installation and find the line about php.ini