Web Programming:

We know how to use PHP to write server-side script to tell web server what to do. We know how to use JavaScript to write client-side script to tell the web browser what to do. In fact, some interactivities such as user input validation, can be written either in PHP or JavaScript.

In your opinion, when would it be more appropriate to use JavaScript or PHP or does it matter which one to use? Support your argument with examples. No actual JavaScript or PHP codes required. As a web developer, why is it important to learn both JavaScript and PPH?