The "the_content" filter is used to filter the content of the post after it is retrieved from the database and before it is printed to the screen.
A plugin (or theme) can register as a content filter with the code:
<?php add_filter( 'the_content', 'filter_function_name' ) ?>
Where 'filter_function_name' is the function WordPress should call when the content is being retrieved. Note that the filter function must return the content after it is finished processing, or site visitors will see a blank page and other plugins also filtering the content may generate errors.
filter_function_name should be unique function name. It cannot match any other function name already declared.
This could be used to provide generated content for a page (as an alternative to the Shortcode_API), or for a set of pages sharing some characteristics (e.g. the same author):
// returns the content of $GLOBALS['post']
// if the page is called 'debug'
function my_the_content_filter($content) {
// assuming you have created a page/post entitled 'debug'
if ($GLOBALS['post']->post_name == 'debug') {
return var_export($GLOBALS['post'], TRUE );
}
// otherwise returns the database content
return $content;
}
add_filter( 'the_content', 'my_the_content_filter' );
This filter function adds an image before the post on the post page (see is_single()). It assumes an image named post_icon.png exists in the theme images folder. It runs at a lower priority (20) which runs later than most other filters (default is 10).
add_filter( 'the_content', 'my_the_content_filter', 20 );
/**
* Add a icon to the beginning of every post page.
*
* @uses is_single()
*/
function my_the_content_filter( $content ) {
if ( is_single() )
// Add image to the beginning of each page
$content = sprintf(
'<img class="post-icon" src="%s/images/post_icon.png" alt="Post icon" title=""/>%s',
get_bloginfo( 'stylesheet_directory' ),
$content
);
// Returns the content.
return $content;
}
Adds a featured image set from the single post Edit screen which displays before the content on single posts only.
add_filter( 'the_content', 'featured_image_before_content' );
function featured_image_before_content( $content ) {
if ( is_singular('post') && has_post_thumbnail()) {
$thumbnail = get_the_post_thumbnail();
$content = $thumbnail . $content;
}
return $content;
}