Function to remove smart quotes, HTML and other special characters from text. Call the function with text as argument, function returns clean text.
1) Copy the file cleanup_text.php to your WordPress plugins directory.
2) Login to WordPress as an administrator, go to Plugins and Activate it.
3) Call the plugin anywhere in your theme or other PHP code.
E.g. replace "the_content()" with "cleanup_text( the_content() )"