There is a difference between being hit by the exploit of a zero-day and being hit by the exploit of something (incorrect security implementations) that has been known to be incorrect for years, decades even. The zero-day in PHP is not your fault (unless perhaps you wrote or maintain that bit of PHP) but bad security implementation using PHP (silly example: not bothering to hash password because it is easier and allows you to allow for typos which is convenient for the user) is ultimately the organisations fault.