Add value to existing field with one query

I want to add a value to a value that is already in the DB.

For example sake this is what I would like to do:

$ValueToAddToDB = 4

UPDATE mytable SET field=add($ValueToAddToDB) WHERE id=555

So in the above example, if "field was already 100, it would now be 104 after the update ran.

Is there a mySQL function like add or sum?

field=field+$valueImAdding

Thanks for posting the answer, I am assuming it works when subtracting or otherwise changing a value, because putting the field name into the right side of the definition makes it like a variable in an equation.

Yep