This error usually looks something like:

Cannot add header information - headers already sent by (output started at /some/path/to/path.php:1)

There are two main times you can see this error:

1. In the first case, this error is caused by blank lines or white space after the closing ?> in the file specified (or before the opening <?php); path.php, in this case.  Edit the file in a regular text editor and remove any white space or blank lines after the closing, or before the opening, PHP tag.

2. If this error appears in addition to another error, then it likely exists simply because the other error is present.  The fact that the other error occurred can cause this error to appear.  Once the other error is eliminated, this error will usually disappear as well.