As Daniel stated, ideally you want a central point in your MVC that handles your database connections. This way it is accessibly in your entire project, plus you can easily keep track of how many connections are running at a time (preferably just one of course).
If you happen to be using a framework, look for a premade library that does these kinds of things. Most recent frameworks have something to handle this.
If you're just creating your own project in native PHP
then you can just implement something that behaves like Daniel described.