To do real parameter handling, use the zend_parse_parameters() function.

You can make it an optional argument by putting a "|" in the argument format string. Like this:

Here are all the possible modifiers in the argument format string: 

'l'  long (integer)
'd'  double (floating point)
's'  string
'b'  boolean
'r'  resource
'a'  array
'o'  any object
'O'  specific object
'z'  zval
'Z'  zval-ref
'|'  next arg is optional
'/'  arg is separated if not a reference
'!'  next arg returns null if type IS_NULL
So, for example, a function that takes a string, an integer, and array and an optional resource would look like this: