3 comments

1. the default filters are set in /wp-includes/default-filters.php;

   for ‘the_content’ this is from line 135:

   add_filter( 'the_content', 'wptexturize'        );
   add_filter( 'the_content', 'convert_smilies'    );
   add_filter( 'the_content', 'convert_chars'      );
   add_filter( 'the_content', 'wpautop'            );
   add_filter( 'the_content', 'shortcode_unautop'  );
   add_filter( 'the_content', 'prepend_attachment' );
   

   to trace the individual functions, you could try and use http://phpxref.ftwr.co.uk/wordpress/nav.html?_functions/index.html
   then follow the links…

   Edit

   Missed one (wp-includes/default-filters.php, line 102):

   // Format WordPress
   foreach ( array( 'the_content', 'the_title' ) as $filter )
       add_filter( $filter, 'capital_P_dangit', 11 );
   

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2. All filters are stored in the global $wp_filter variable. You can inspect that variable to see which functions are bound to a certain filter.

   global $wp_filter;
   print_r($wp_filter['the_content']);
   

   To exactly understand the array that gets outputted, knowing how the $wp_filter variable is built up helps a lot. This line is from the source of the add_filter() function:

   $wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
   

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3. but what does the_content filter do?

   The content_filter enables you to return custom content before or after the content.

   2 very basic practical examples of using the_content filter in a custom function are as follows:

   This example returns custom content before the content

    add_filter( 'the_content', 'return_before_content' ); 
   
    function return_before_content( $content ) { 
   
       if ( is_singular('post')) {
   
           $before_content = '<p>Text Returned Before the_content</p>';
   
           $content = $before_content . $content;
   
           }
   
       return $content;
   }
   

   This example returns custom content after the content

    add_filter( 'the_content', 'return_after_content' ); 
   
    function return_after_content( $content ) { 
   
       if ( is_singular('post')) {
   
            $after_content = '<p>Text Returned After the_content</p>';
   
           $content = $content . $after_content;
   
           }
   
       return $content;
   }
   

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