Append a line to a Text File

I still need to know how to do it, since the "a+" or the "a" give me problems... I need to be able to add a line at the end of a text file thru sending the information (and by this I mean the e-mail to be writen) thru a form, and this new information should be added to the file at the end of it without overwriting it. Thanx, Rodrigo PS: If possible send a code for instruction. ;)