You generally receive a:

Cannot modify header information - headers already sent by...

when a php script returns output to the screen (such as html code) and then tries to modify any of the header information. Headers should always be sent before any other ouput (such as html), and php will throw an error message if headers are sent after any other data is sent.

In this article, we'll provide two generic examples to show what is happening.

Example 1:

Example 2:

In example 2, we have two files:

2.php

3.php

In this example, 2.php includes the logic that would validate a login attempt, and 3.php would take action. It is a very simply