You may get this error on a PHP page:

 Warning: session_start(): Cannot send session cookie - headers already sent

Change the following setting in your php.ini file.  This should be initially off, turn it on like so:

output_buffering = On

This should resolve the issue if it doesn't you ensure that you have NO Blank lines or other characters before session_start();

Meaning it should look like this:

<php?
session_start();

YOUR CODE

You FTP and download the file and open it with notepad, make sure there are no lines or spaces before the <php? starting tag or after the ?> closing tag.  Save the file as "ANSI" encoding, not UTF-8 or anything else via notepad

File -> Save As

Change the file type to "All Files *.* " and change the encoding type to ANSI, name your file (Something.php) upload it back to the appropriate directory.

Also, check your MySQL connection strings, this may cause this issue as well.