Define default values for function arguments

In the Lua wiki I found a way to define default values for missing arguments:

function myfunction(a,b,c)
    b = b or 7
    c = c or 5
    print (a,b,c)
end

Is that the only way? The PHP style myfunction (a,b=7,c=5) does not seem to work. Not that the Lua way doesn't work, I am just wondering if this is the only way to do it.