PHP Form Validation - with PHP4+ and register_global = off

The validation of data that has been entered in a form is necessary in most cases. Why is important? For example, what good is holding a contest or sweepstakes if you can't notify the winner, because he or she entered an invalid telephone number or an incorrect address. What good is having a mailing list if the e-mail addresses on it aren't verified, and your mailing list just bounces back to you without reaching the subscribers and target audience.<br /><br /> Validating form entries saves you time and more importantly, it can save you money. And since somebody embossed the slogan "Time is money!", this should be very important for your web site!<br /><br /> Well when should we validate? There are two types of validation; client side and server side.