Creating a plugin: do I need unique variable names?

I read somewhere that I need to make sure that any functions I write in my plugin need to have unique names so that they don’t conflict with other plugins, so I’m prefixing my functions with an abbreviation of the plugin name. But I can’t find whether or not the same is true for variables? Can I simply have $a and $b or should I make them something like $afift_a and $afift_b so that they are unique?