@ trisweb: that's right, thanks that's saved me a lot of times trying to scan the codes.
So to conclude: One of the way to avoid displaying:
Warning: Cannot modify header information - headers already sent by (output started) etc.
1- As mentionned, go at the top and the bottom of the code and clear the space before and after the <? ?>
2- Re-check the last page code you were working on (because it's where usually you will found 80% of the code problems) you did certainly put an ' instead of " somewhere, or forgot to add \n on a line or whatever.