The vulnerability stems from inline code in the 'tests/test.php' script that processes all GET parameters without proper validation, allowing attackers to inject arbitrary variables. The code in question is a foreach loop in the global scope (not within a named function) that blindly assigns user-supplied GET parameters to variables (using variable variables: $$k = $v). The lack of input validation here directly enables XSS. Since the vulnerable code is not encapsulated within a specific function but exists in the script's main execution flow, no named functions are identified as vulnerable with high confidence.