get thumbnail image from first image

function custom_get_post_thumbnail_id( $thumbnail_id, $post ) {
    if ( ! $thumbnail_id ) {
        if ( $post && 'post' === $post->post_type ) {
            preg_match_all( '/<img[^>]+>/i', $post->post_content, $img_matches );

            if ( isset( $img_matches[0] ) && count( $img_matches[0] ) > 0 ) {
                $first_img = $img_matches[0][0];
                preg_match( '/src="([^"]+)/i', $first_img, $img_src );
                $img_url         = str_ireplace( 'src="', '', $img_src[0] );
                $img_url_no_size = preg_replace( '/-\d+x\d+(?=\.(jpg|jpeg|png|gif))/i', '', $img_url );
                $attachment_id   = attachment_url_to_postid( $img_url_no_size );
                if ( $attachment_id ) {
                    return $attachment_id;
                }
            }
        }
    }

    return $thumbnail_id;
}
add_filter( 'post_thumbnail_id', 'custom_get_post_thumbnail_id', 10, 2 );