How to call a function from a string stored in a variable?

I need to be able to call a function, but the function name is stored in a variable, is this possible? e.g:

function foo ()
{
    //code here
}

function bar ()
{
    //code here
}

$functionName = "foo";
// I need to call the function based on what is $functionName

Answer

$functionName() or call_user_func($functionName)