Where does it come from ?

Most of the time, it is just a matter of warnings sent during the activation of the plugin. For instance, when you set WordPress in debug mode (WP_DEBUG to true) or when you use xDebug.

How to deal with that ?

We set WordPress WP_DEBUG to false, we disable xDebug, and we play with error_reporting flags in our php.ini in order to hide warnings. Ok, I’m joking ! It might maybe be a good idea to fix our warnings. But how to display those warnings, because blind debugging is a pain.

How to access to these characters ?

Well, we are gonna play with output buffering. What’s that ? It’s exactly how it sounds, it buffers the characters that were meant to be display on the output. We will use two php functions ob_start and ob_get_contents. The first one starts to buffer the data, while the second one just gets what’s inside this buffer.

So it’s gonna look like that:

</p>
<p>register_activation_hook(__FILE__,'myplugin_activate');</p>
<p>function myplugin_activate(){</p>
<p>ob_start();</p>
<p>// Here your plugin activation code</p>
<p>$contents = ob_get_contents();</p>
<p>}</p>
<p>

For getting $contents, I let choose: set a breakpoint and debug with your favorite tool, a little exit or even error_log.

There is a bunch of links that google can give you about output buffering if you want to know more about that.

Have fun with it !