How can global variables be manipulated or overridden by users in PHP?

Global variables in PHP can be manipulated or overridden by users through user input, such as form submissions or query parameters. To prevent this, it is important to sanitize and validate all user input before using it to modify global variables. One way to achieve this is by using PHP's filter_input() function to filter and validate input data.

// Sanitize and validate user input before using it to modify global variables
$user_input = filter_input(INPUT_POST, 'user_input', FILTER_SANITIZE_STRING);

// Assign sanitized user input to a global variable
$GLOBALS['global_variable'] = $user_input;