Question about how do wordpress filters/actions work

For every filter or action hook that you use with the add_filter or add_action functions, there is a corresponding apply_filters or do_action function called somewhere in the WordPress core. This function sets up which parameters will be sent to the filter or action.

For the filter hook the_content example that you gave, the filter hook is used in the function the_content found in wp-includes\post-template.php in WordPress 3.7.1:

function the_content( $more_link_text = null, $strip_teaser = false) {
    $content = get_the_content( $more_link_text, $strip_teaser );
    $content = apply_filters( 'the_content', $content );
    $content = str_replace( ']]>', ']]&gt;', $content );
    echo $content;
}

In the apply_filters function call, the first parameter is the hook name, in this case 'the_content'. The next parameter, $content, is the value that we want to filter. This will be the first parameter in your filter function (which is named $content in your my_the_content_filter function, but it could have any name). Optionally, more parameters can be specified in the apply_filters function, and these will be passed in additional input parameters to the filter function.

For a filter hook, your filter function returns a filtered value, which in turn is returned by the apply_filters function. For action hooks, there isn’t any return value used.

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