php - What do these NULLS mean? -

from page on @ microsoft; http://msdn.microsoft.com/en-us/library/cc626305.aspx, give examples on how use paramaterized queries showing how build array. here snippet of sample code:

$params1 = array(                array($employeeid, null),                array($changedate, null, null, sqlsrv_sqltype_datetime),                array($rate, null, null, sqlsrv_sqltype_money),                array($payfrequency, null, null, sqlsrv_sqltype_tinyint)            ); 

i understand variables , constants, fail explain nulls for. used, aren't , there two;

variable, null, null, constant 

can explain me, or point me right place? can't find reading material this.

thank you.

this akin invoking overloaded function. function takes 2 parameters:

       array($employeeid, null)   

sometimes 4:

           array($rate, null, null, sqlsrv_sqltype_money) 

placing null in these function calls means no value. if @ different queries/functions invoked these see parameters used in different ways. further null passed function implies optional parameter, why comes in no value or null.