PHP and MySQL for Dynamic Web Sites: Visual QuickPro Guide

Validating Form Data

A critical concept related to handling HTML forms is that of validating form data. In terms of both error management and security, you should absolutely never trust the data being entered in an HTML form. Whether it’s purposefully malicious or just unintentionally inappropriate, it’s up to you—the Web architect—to test it against expectations.

Validating form data requires the use of conditionals and any number of functions, operators, and expressions. One common function to be used is isset(), which tests if a variable has a value (including 0, but not NULL or FALSE).

if (isset($var)) {
   // $var has a value.
} else {
   // $var does not have a value.
}

One problem with the isset() function is that an empty string tests as ...