The Plugin Generated x Characters of Unexpected Output During Activation

While developing a new WordPress plugin I ran into a common problem when the plugin was activated. The following message popped up:

The plugin generated 1616 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

The most common causes are:

1. A white space before or after the PHP opening or closing tags
2. A file encoded in UTF-8
3. Another issue when something is called at the wrong time, or a call that cannot be resolved without intervention
4. Using the WordPress add_option function. Switching to update_option instead can resolve the problem.

The first is the most common mistake for new programmers. Once the space is removed before or after the PHP opening and closing tags try deactivating and reactivating the plugin to see if the message is gone.

The second stumped me. I use several local development servers including Uniform Server, WAMP Server, and XAMPP. The files were encoded in UTF-8, the universal language encoding that is the standard, but the server threw errors. I finally converted the files to ANSI encoding, and the problem was fixed. Some web hosts also run software that generate this error, so it is best to encode the plugin in ANSI to avoid unexpected problems.

If the first two causes have been eliminated, and the problem persists then this issue needs to be tracked down and eliminated, otherwise it might cause problems. To save the error message to debug the problem the following code snippets can help:

function tl_save_error() {
    update_option( 'plugin_error',  ob_get_contents() );
}
add_action( 'activated_plugin', 'tl_save_error' );

Then to display the error message:

echo get_option( 'plugin_error' );

Or you could do the following:

file_put_contents( 'C:\errors' , ob_get_contents() ); // or any suspected variable

If you have another method to save and display these error messages share them.