Sameer has posted his "easy way" to validate user input coming in over a POST request:

Validating POST data from a form is a common requirement for a developer. If the number of form fields are few than the validation is a small matter. But the case is different when the form contains more than 15 or 20 fields and some of the fields are mandatory. The following code will give you an idea of how to easily validate mandatory fields, whatever the number of fields.

His method prefixes the form fields with a certain string (in his case "c_") and uses that to loop through and act as a hook to check only the form values that were submitted (and nothing else that happens to be in the $_POST array). Any number of checks could be added on to this simple example including type checks, length and validating off of another field - like a password confirm match.