Add the following code to your functions.php file.

To run it, simply visit any page on the site – DO NOT USE THIS CODE ON YOUR LIVE SITE! Any visitor that visits any page will trigger this code. This code was created to manipulate content on a development site before it was merged with the live site

/** Function to set the first image as featured **/
function set_first_image_as_featured() 
{
/** Set the post arguments, here we're targetting all posts in the
	To go through all posts 
**/
  $args = array(
    'post_type' => 'post',
	'category_name' => 'articles',
    'numberposts' => -1, // Modify this to limit the number of posts if needed
  );

  $posts = get_posts($args);

/** Force include image files so that we can download and
    save the images locally  
**/
if(!function_exists('wp_get_current_user')) {
include(ABSPATH . "wp-includes/pluggable.php"); 
}
require_once(ABSPATH . 'wp-admin/includes/media.php');
require_once(ABSPATH . 'wp-admin/includes/file.php');
require_once(ABSPATH . 'wp-admin/includes/image.php');

/** Loop through all posts **/

  foreach ($posts as $post) {
  /** Only run if we don't already have a thmbnail image set **/
 if (!has_post_thumbnail($post->ID)) 
 { 

    $content = get_post_field('post_content', $post->ID);
	$alttext = get_post_field('post_title', $post->ID);
	/** Check we have post content to check **/
	if($content!='')
	{
	
	$doc = new DOMDocument();
    @$doc->loadHTML($content);
	/** Grab All Image Elements in the DOM. You can change 'img' to any
	    HTML tag such as 'p', 'iframe' etc - amend the following code
             according to your needs
	**/
    $images = $doc->getElementsByTagName('img');
	
	/** Check to make sure we have at least one image in the post **/
    if ($images->length > 0) 
	{
		/** Get the URL of the first image **/
      $first_image_url = $images->item(0)->getAttribute('src');
	  
     /** Check if image URL starts with an external domain (not your own website)**/
     if (strpos($first_image_url, get_home_url()) === false) {
		
        /** WARNING: Downloading from external sources can have copyright and security risks.**/
		
		/** Add the image to the WordPress Media Library and grab the id of the insered image
		    So we can assign it as the post thumbnail later
		**/
        $featured_image_id = media_sideload_image($first_image_url, $post->ID, $alttext, 'id');
	   
		/** Check whether we have a valid image id so we can attach it to the post **/
        if (!is_wp_error($featured_image_id)) 
		{
			/** Set the post thumbnail with the first image in the post **/
			set_post_thumbnail($post->ID, $featured_image_id);
			
			/** Remove the image from the post content and save the amended content
				Comment Out the line below if you want to keep the image in the post
			**/
			update_post_content_with_image_removed($post->ID, $first_image_url);
			
			/** Uncomment the following line for visual confirmation of items updated **/
			//echo 'done '.$post->ID.' IMG ID '.$featured_image_id. ' IMG - '.$first_image_url.' ALT - '.$alttext.'<br>';
        } // End Set post thumb
		
      } // END External Image Check
    } // END Has in post image
  }// End has Content
}// END Has Thumbnail
} // END Post Loop

} // End function

/** Run the function after WordPress is loaded **/
add_action('init', 'set_first_image_as_featured');

function update_post_content_with_image_removed($post_id, $image_url) 
{
  // 1. Fetch the existing post content
  $existing_content = get_post_field('post_content', $post_id);

  // 2. Escape the image URL for safe use in string replacement
  $escaped_url = str_replace('/', '\\/', $image_url);

  // 3. Remove the '<img>' tag with the specified URL using regular expressions
  // This approach is more flexible than str_replace for potential variations in the tag
  $updated_content = preg_replace('/<img[^>]*?src="' . $escaped_url . '"[^>]*?>/i', '', $existing_content);
  
  // 4. Update the post content in the database
  $args = array(
    'ID' => $post_id,
    'post_content' => $updated_content,
  );

	// Update the post with the revised content
		wp_update_post($args);
  
} // END update_post_content_with_image_removed