Fusion Defender is a the core function written to halt functions codes from execution by constantly processing error constant return "constant: FUSION_NULL" should bad things or unintended things happen to safeguard against any vulnerabilities on your files when it is being used. And if may, it is our own anti-non-standard-codes protection.
By default, it is invoked by Maincore.php to check against Cookies, SESSIONS and CSRF tokens.
Later, it is called by every single form_sanitizer() function during validation of fields.
PHP-Fusion Defender was first introduced in the Version 9.00.