php.ini is a configuration file that is read when PHP script starts up. PHP looks for the file in the current working directory, in the path designated by the environment variable PHPRC, and in the path that was defined in compile time (in that order). php.ini is usually located in the root directory where your PHP package is installed.
php.ini contains configuration settings that control the way PHP scripts are interpreted. It includes directives and settings that control issues like security, performance, data and file handling, etc. It also indicates whether certain PHP extensions should enabled on your system.
PHP Manual - php.ini