That site is written by geeks for geeks so isn't easy for the layman to understand but is invaluable for anyone wanting to get into PHP.

The problem isn't so much an OSC one but a PHP one. To resolve it, you'll probably have to open each page (not every file though) in your editor and save them without BOM encoding. In Dreamweaver, you can set this from edit/preferences. All the actual pages will need to be re-saved and any files that are called before the session_start() function is invoked such as sessions.php and configure.php. If you miss one, the error message will display which file is now causing the error, simply go though each one as it appears and save without BOM. That should rid you of the problem.

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Lawrence