It sounds like you may not have uploaded the most recent version of the page:
webassist/form_validations/wavt_scripts_php.php
The function is says is missing was added in this version of securityassist, so if that file wasn't updated it would generate the error you are reporting. Try uploading that file... if that doesn't work I have attached the newest version of that file that you can use to replace the version you have now which should correct that error.