Client-side validation is very useful, and saves a lot of time. However, if there is no server-side validation to make sure that the input is correct, then there is no guarantee that the data you collect will be correct.
In this case, PHP is very useful to us to manage the form. When you think of it, the form would be defined in the following three areas:
bodysection in the HTML form
If any one of these was different to the rest, then there would be problems.