Understanding of both server-side and client-side is important. Client side is in-secure, transparent and can be manipulated by user (or hacker) in any way. Server-side is protected and can't be messed around with. Any web application with user log-ins should have back-end code.
You might be able shift your code based on your requirements. For example, you can place your code primarily on the client side with Sencha / ExtJS or similar framework, then have only a simple API service on your server. You might even avoid coding for the backend completely by using some API engine.
On other hand, you can stay completely on the server-side if you pick a PHP UI Framework. Such a framework would worry about your user interface letting you use PHP only. Unlike languages such as Java and .NET There are very few UI Frameworks for PHP. I wish they would have more publicity.
Coding for both front-end and back-end makes development much slower, but it's a commonly accepted way right now.