PHP.ini is a configuration file that is used to customize behavior of PHP at runtime. This enables easy administration in the way you administer Apache web server using configuration files. The Settings like which upload directory, register global variables, display errors, log errors, max uploading size setting, maximum time to execute a script and other configurations is written in this file.
When PHP Server starts up it looks for PHP.ini file first to load various values for settings. If you made changes in PHP.ini then you need to restart your server to check the changes be effected. Default php.ini file of web server.the copy of this ini file can be found in /usr/local/lib/php/ for UNIX