Warning: Cannot modify header information - headers already sent
Happens when your script tries to send an HTTP header to the client but there already was output before, which resulted in headers to be already sent to the client.
This is an E_WARNING and it will not stop the script.
A typical example would be a template file like this:
<html>
<?php session_start(); ?>
<head><title>My Page</title>
</html>
...
The session_start() function will try to send headers with the session cookie to the client. But PHP already sent headers when it wrote the <html> element to the output stream. You'd have to move the session_start() to the top.
You can solve this by going through the lines before the code triggering the Warning and check where it outputs. Move any header sending code before that code.
An often overlooked output is new lines after PHP's closing ?>. It is considered a standard practice to omit ?> when it is the last thing in the file. Likewise, another common cause for this warning is when the opening <?php has an empty space, line, or invisible character before it, causing the web server to send the headers and the whitespace/newline thus when PHP starts parsing won't be able to submit any header.
If your file has more than one <?php ... ?> code block in it, you should not have any spaces in between them. (Note: You might have multiple blocks if you had code that was automatically constructed)
Also make sure you don't have any Byte Order Marks in your code, for example when the encoding of the script is UTF-8 with BOM.
Related Questions:
Do I have pay attention to these messages?
No, you can safely ignore them.
What does this message mean?
See this answer.
Best Answer
This is a bug in Qt Creator, which is not invoking GDB correctly (either inside a pseudoterminal, or with command line arguments that tell it not to expect to be run inside a pseudoterminal). It is also a bug in GDB, which could figure out for itself that it wasn't being run inside a pseudoterminal and behave accordingly. I suspect the GDB maintainers will take the position that this is Qt Creator's fault, and vice versa, alas.
There is a claimed workaround here: http://www.qtforum.org/article/31905/debugging-qt-application-on-linux.html but it sounds kinda dodgy to me.