This vulnerability occurs when you accept user input and then use that input in output. The solution is to sanitize the input. The filter_var or filter_input functions can be used with a variety of sanitize filters.

As an example, code to accept what should be an integer value from user input:

     $iVariable = $_POST['someUserInput'];

Becomes code that removes all characters except for digits (and + or – signs) from the input:

     $iVariable = filter_input(INPUT_POST, 'someUserInput', FILTER_SANITIZE_NUMBER_INT);