PHP.INI file is the configuration file of PHP in which the PHP settings are kept.
Changing / Edit php.ini file
In a shared hosting environment you need to contact your host to change the settings of this file. Any change in php.ini is applicable to whole server ( not only to your website ) so your host is unlikely to agree for changes all the time.
How to read the settings of php.ini file
Best way is to use phpinfo function. Single settings can be read by using ini_get function.
Some settings we can change at script level by using the function ini_set() function.
How to change or locate in my localhost
Search for the php.ini file. It will be there in your PHP installed directory or inside windows directory. Open this file in note pad or any other text editor and do the changes. You may need to restart your PC to reflect the changes. You can check phpinfo to verify the changes.
You may need some extensions to be added or to be removed. You can do all these changes at php.ini file. While installing PHP you based on your selection these extension will be added or removed. After installation also you can change these settings.