Can I share variables between different functions in PHP?

I'll try to explain with an example...

Let's say I have two different functions, and one of them has a defined variable. In the second function, I don't wanna write the same variable again, can I simply use the variable from the first function in the second one WITHOUT redefining it in the second function?

Something like:

function a()
{
  $var = "my variable";
}

function b()
{
 echo $var;
}

Sorry if this questions is a bit silly, but I'm still a beginner =).