One of the key ingredients to modern Web applications is interactivity—the ability to accept input from users, which helps shape their experience. That input can be just about anything, from a simple search term to entire user-submitted novels. The key is the ability to process this input and turn it into a meaningful feature that enriches the experience for all the users of the site.

The process begins by sending an HTML form to the Web browser, where a user can fill it in and submit it back to the server. When the data arrives, it must be validated to make sure the user didn't forget any fields or enter anything inappropriate. If there was anything wrong with the submitted data, it has to be sent back to the user for corrections. ...

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