I am creating a custom backend for a custom project management system for my company. I am using HTML5, PHP, and MySQL to develop this backend.

I am only unaware how to create one section of my outline. I do not know what language is required to create an Instant Messenger between my users??

What is the easiest way to add Instant Messenger between my users into my custom backend?

What language should I use to create Instant Messenger?

Any advice is appreciated!

Thank you