ZenCart Warning: Cannot modify header information

ZenCart Warning: Cannot modify header information
After installing Zencart you may receive a warning saying “Cannot modify header information headers already sent by…” in your zen cart.

Cannot modify header information headers already sent by (output started at <path/to/output_file>:<line number>) in <path/to/current_file> on line <number>.
This warning is encounter after modifying some .php files. This warning shows up if you leave a space or extra line at the beginning, or the end of a .php file. Usually the file causing such problem is indicated by its name and line number. All you need to do is Open that file and go to the line number as reported in the error message. Then make sure that that there is no space or extra line is present before <?php or after the ?> tag.
This error usually occurs when you are working with some custom editors such as Dreamweaver. Sy for example, you have tried editing files using Dreamweaver and uploaded the files via Dreamweaver without using check-in features. You can also avoid this error by using check-in features in Dreamweaver.