commas

Is there a function that will strip or add commas? I'm trying to do calculations with attendance numbers of football games... for example: 33,153 right now I don't have a comma for the numbers in the database. So the above example is 33153 in my database. If I put the comma in, when I try to do calculations with php it sees it as 33 instead of 33,153. So is there a way to add that comma afterwards? Perhaps a string function at will put the comma in after the first 3 characters? Thanks