@NogDogMar 19.2013 — #JavaScript validation would only be for user convenience (which is not a bad thing, mind you), but it is easily bypassed, so you still have to do any "important" validation on the server side.
@NogDogMar 20.2013 — #Because if it's something like validating that a field is an integer between 1 and 100, you can validate that on the browser side (JavaScript) and not have to send a HTTP request to the server, wait for the response, and then redisplay the page with the desired error message when you find out they entered
"-123.59". Not a big deal, and something that can be added when you have time, or never if you don't feel it's important and server response time is seldom an issue on your site -- just another tool in your toolkit when it makes sense to do it. (And if you code your JavaScript every bit as modularly as you code your PHP and make use of JavaScript frameworks where it makes sense to do do, then it does not have to take all that much work to do -- beyond the learning curve of using a new framework and such.) Like pretty much everything else in this business, there's never one correct answer: it all depends on specific requirements, time, and budget.