Making a parameter in a function optional

[YourNameHere]

How would a make a function that "CAN" take 4 params but make 2 optional. In some of the language construct functions I see it is possible.  but I get an error when I do it with a homemade  function.

 

for example: mysql_error();  will take two params but you can omit the $link identifier.

[Mchl]

function func($v1,$v2,$v3 = null, $v4 = null) {
...
}

[YourNameHere]

function func($v1,$v2,$v3 = null, $v4 = null) {
...
}

 

Awesome thank you!