Need the proper syntax to set a value of a field.

Need the proper syntax to set a value of a field by calculated the values of 2 0tger fields in my server side PHP.

Here is what I have tried.

        Field::inst('xxxx.Fld1),
        Field::inst('xxxx.Fld2),

        Field::inst('xxxx.Fld3)
               ->setValue( Field:: Fld1 * Field:: Fld2 )