empty
The empty function sets the value of a field in internal storage to null. This function is not the same as setting the value of a field to a zero-length string (" ") or to zero.
Syntax
Use this syntax:
empty($GROUP_NAME[index][index][index]. #FIELD_NAME)
Example
An example of this function follows:
empty($GROUPNAME[0][1][1].#FIELDNAME);
//Set the value of the specified instance of the FIELDNAME field to //null