I need to make a multi-vendor e-commerce website by integrating Indian payment gateways. Which language apart from HTML, CSS, and PHP should I learn?

If you were to ask this question many years ago, my answer would be that learning HTML / CSS and PHP is enough for a dynamic shopping website.

However, with the advent of AJX (Asynchronous JavaScript and XML), it has become a common practice to automatically retrieve data without reloading an entire web page. The importance of JavaScript as a web technology can be measured by the fact that 94.5% of all websites are currently using it. As a client-side programming language, JavaScript helps web developers make web pages dynamic and interactive by implementing custom client-side scripts. John Resig developed the initial version of jQuery in 2005 and released it in 2006 at an event called BarCampNYC. With thousands of prebuilt plugins that you can use to help you develop your own website, Ajax allows you to make asynchronous calls to a web server. This allows the client browser to avoid waiting for all data to arrive before allowing the user to work again.

You mentioned “Payment Gateway” integration. Nowadays, many payment gateways use Jason or the XML API. So, I think some knowledge about Jason and XML would be helpful.