How can the use of register_globals in PHP impact the security and functionality of a web application?

The use of register_globals in PHP can impact the security of a web application by making it vulnerable to security risks such as variable injection attacks. It can also affect the functionality of the application by causing unexpected behavior due to global variables being automatically created from user input. To mitigate these risks, it is recommended to disable the use of register_globals in PHP by setting it to "Off" in the php.ini configuration file or using the ini_set() function within the application.

// Disable register_globals in PHP
ini_set('register_globals', 'Off');