pp-wp-snippet-delete-thumbnail.php

<?php
// This code snippet automatically deletes the thumbnail if a post is deleted

// Add a custom function that runs before a post is deleted
add_action( 'before_delete_post', 'remove_post_thumbnail', 10 );

// This function deletes the image assigned as thumbnail from the media library
function remove_post_thumbnail( $post_id ) {
  
  // First we have to check, of there's a thumbnail assigned to the post
  if( has_post_thumbnail( $post_id ) ) {
    
    // OK, this post has a thumbnail
    // Let's get the ID
    $attachment_id = get_post_thumbnail_id( $post_id );
    
    // Before we can delete it, we have to check if it used anywhere else
    
    // First we'll check if it is used as a thumbnail by another post
    if ( empty ( get_posts( array( 'post_type' => 'any', 'post_status' => 'any', 'fields' => 'ids', 'no_found_rows' => true, 'posts_per_page' => -1, 'meta_key' => '_thumbnail_id', 'meta_value' => $attachment_id, 'post__not_in' => array( $post_id ) ) ) ) ) {
      
      // The image is not used as thumbnail except by the post we want to delete
      
      // Now we have theck if it's used somewhere in content - this is a bit more of work...
      
      // We need the image URLs for all sizes
      $attachment_urls = array( wp_get_attachment_url( $attachment_id ) );
      foreach ( get_intermediate_image_sizes() as $size ) {
        $intermediate = image_get_intermediate_size( $attachment_id, $size );
        if ( $intermediate ) {
          $attachment_urls[] = $intermediate['url'];
        }
      }
      
      // Now we can search for these URLs in content
      $used = array();
      foreach ( $attachment_urls as $attachment_url ) {
        $used = array_merge( $used, get_posts( array( 'post_type' => 'any', 'post_status' => 'any', 'fields' => 'ids', 'no_found_rows' => true, 'posts_per_page' => -1, 's' => $attachment_url, 'post__not_in' => array( $post_id ) ) ) );
      }
      
      if ( empty( $used ) ) {
        // The image is nowhere used in content
        // So finally we can delte it
        wp_delete_attachment( $attachment_id, true );
      }

    }

  }

}

?>