Compatibility with Dreamweaver CS6

Folks, I am new to web development but learning fast and have a basic question. I have a website that used “Dreamweaver CS6” to build the site and there have been several Dreamwaeaver “Extensions” used in the development of the site. 

Additional information (if it helps)

• Languages used are HTML, PHP
• the query language used is MYSQL along with a little javascript. 
• The styling used is “Bootstrap 3.2” (CSS).
• There is a custom CSS style sheet that sets some styles that bootstrap does not cover (public view pages).
• Styling in the administration area is stored in custom built CSS files 

My question

I have been advised by the original developer (non IT expert - learned by himself) that I need Dreamweaver 6 to carry on maintaining the code. Can someone confirm for me if that is correct and if so where I can get this version as I suspect it is very old. Or which readily available version provide backwards compatibility?