AJAX and PHP

4
AJAX Form Validation
Validating input data is an essential requirement for quality and secure software applications. In
the case of web applications, validation is an even more sensitive area, because your application is
widely reachable by many users with varying skill sets and intentions.
Validation is not something to play with, because invalid data has the potential to harm the
application's functionality, and even corrupt the application's most sensitive area: the database.
Input data validation means checking whether the data entered by the user complies with
previously defined rules, which are established according to the business rules of your application.
For example, if you require dates to be entered in the YYYY-MM-DD format, then a date of
"February 28" would be considered invalid. Email addresses and phone numbers are other
examples of data that should be checked against valid formats.
Carefully define the input data validation rules in the software requirements document of
the application you're developing, and then use them consistently to validate your data!
Historically, web form validation was implemented mostly at the server side, after the form was
submitted. In some cases, there was also some JavaScript code on the client that performed simple
validation such as checking whether the email address was valid, or if a user name had been entered.
The problems encountered with traditional web form validation techniques are:
• Server-side form validation meets the limits of the HTTP protocol, which is a
stateless protocol. Unless special code is written to deal with this issue, after
submitting a page containing invalid data, the user is shown back an empty form that
has to be filled from scratch.
• When submitting the page, the user needs to wait for a full page reload. For every
mistake that is made when filling the form, a new page reload happens.
In this chapter, we will create a form-validation application that implements the good old traditional
techniques and adds an AJAX flavor, thereby making the form more user-friendly and responsive.
Even if you implement AJAX validation, server-side validation is mandatory, because the server is
the last line of defense against invalid data. The JavaScript code that gets to the client can not only be
disabled permanently from the browser's settings, but it also can be easily modified or bypassed.
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