1. Short answer to the 5th question: A closure in JavaScript is a function that is defined within another function. It's a great way to create private variables and to encapsulate logic that you want to keep out of your main application structure.

2. Short answer to the 7th question: To optimize website performance, you need to make sure that your website is as lightweight as possible. This means using lightweight HTML and PHP templates, and finding ways to reduce the amount of JavaScript and CSS code on your pages.

3. Short answer to the 9th question: Responsive design is a approach to designing and developing websites where the content and layout responds to the size of the screen it's being viewed on. This is achieved by using a combination of CSS media queries and flexible web layouts - the content and layout are tailored to the size of the screen, and the page can be automatically resized to display correctly on different devices. Responsive design makes it easier for your users to interact with your website, and it also improves overall usability and accessibility.
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