How do you create optional arguments in php?

Question

In the PHP manual, to show the syntax for functions with optional parameters, they use brackets around each set of dependent optional parameter. For example, for the date()

Answer 1

The default value of the argument must be a constant expression. It can't be a variable or a function call.

If you need this functionality however:

function foo($foo, $bar = false)
{
    if(!$bar)
    {
        $bar = $foo;
    }
}

Assuming $bar isn't expected to be a boolean of course.