Automatically set featured images

functions.php

<?php

function auto_featured_image() {
    global $post;

    if ( ! has_post_thumbnail( $post->ID ) ) {
        $attached_image = get_children( "post_parent=$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) ;
              }
         }
    }
}
// Use it temporary to generate all featured images
add_action( 'the_post', 'auto_featured_image' );
// Used for new posts
add_action( 'save_post', 'auto_featured_image' );
add_action( 'draft_to_publish', 'auto_featured_image' );
add_action( 'new_to_publish', 'auto_featured_image' );
add_action( 'pending_to_publish', 'auto_featured_image' );
add_action( 'future_to_publish', 'auto_featured_image' );