Want to populate the end date of the month in php

Want to populate the end date of the month in php

I have a two datefield textbox. In one field, i populate it by getDate()
which returns the current date . But in another table i want to populate
the exact the last date of that month.
For example Current date : 9/20/2012 in one text Now next date must be
9/30/2012
Similarly Current date : 01/14/2012 in one text Now next date must be
01/31/2012