Automatically Set Post Featured Image

A post featured image was introduced in Wordpress version 2.9, it is a thumbnail that can be set to the post as a featured image of your post. It will not be displayed on the page but it's up to the Wordpress theme to decide when to display it.

#Enable Theme Support For Featured Post

By default themes do not support this feature you need to add some code to the functiona.php file.

add_theme_support( 'post-thumbnails' ); 

#Set Featured Image

There is a meta box on the post screen where you can set an image to be the featured image on the post.

#Automatically Set Featured Image

But if you forget to place a featured image on your post then your theme won't be able to display an image. Here is a snippet which will automatically set the first image from the post to be your featured image.

<?php
function autoset_featured() {
          global $post;
          $already_has_thumb = has_post_thumbnail($post->ID);
              if (!$already_has_thumb)  {
              $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
                          if ($attached_image) {
                                foreach ($attached_image as $attachment_id => $attachment) {
                                set_post_thumbnail($post->ID, $attachment_id);
                                }
                           }
                        }
      }
add_action('the_post', 'autoset_featured');
add_action('save_post', 'autoset_featured');
add_action('draft_to_publish', 'autoset_featured');
add_action('new_to_publish', 'autoset_featured');
add_action('pending_to_publish', 'autoset_featured');
add_action('future_to_publish', 'autoset_featured');
?>

#Change The Set Featured Image Text

To change the text in the set featured image meta box you can use the Wordpress filter admin_post_thumbnail_html and replace the content with the new text.

function change_featured_image_text( $content ) {
    return $content = str_replace( __( 'Set featured image' ), __( 'Set post thumbnail image' ), $content);
}
add_filter( 'admin_post_thumbnail_html', 'change_featured_image_text' );