How to setup wordpress default featured image

gistfile1.php

<?php
add_filter( 'get_post_metadata', 'WDFI_get_post_metadata', 50, 10);
function WDFI_get_post_metadata($null, $object_id, $meta_key, $single ){
	        if ( ( is_admin() && ( ! defined( 'DOING_AJAX' ) || ! DOING_AJAX ) ) ) {
	                return $null;
	        }
	        if ( ! empty( $meta_key ) && '_thumbnail_id' !== $meta_key ) {
	                return $null;
	        }
	        $post_type = get_post_type( $object_id );
	        if ( false !== $post_type && ! post_type_supports( $post_type, 'thumbnail' ) ) {
	                return $null; 
	        }
	        $meta_cache = wp_cache_get( $object_id, 'post_meta' );
	        if ( ! $meta_cache ) {
	                $meta_cache = update_meta_cache( 'post', array( $object_id ) );
	                if ( ! empty( $meta_cache[ $object_id ] ) ) {
	                        $meta_cache = $meta_cache[ $object_id ];
	                } else {
	                        $meta_cache = array();
	                }
	        }
	        if ( ! empty( $meta_cache['_thumbnail_id'][0] ) ) {
	                return $null;
	        }
	        // add attachment id here
	        $default_thumbnail_id =  1;
	        $meta_cache['_thumbnail_id'][0] = $default_thumbnail_id;
	        wp_cache_set( $object_id, $meta_cache, 'post_meta' );
	        return $null;
	}
  
  add_filter( 'post_thumbnail_html', 'WDFI_post_thumbnail_html', 100, 10);
  function WDFI_post_thumbnail_html( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
          // add attachment id here
	        $default_thumbnail_id =  1;
	        if ( (int) $default_thumbnail_id !== (int) $post_thumbnail_id ) {
	                return $html;
	        }
	        if ( isset( $attr['class'] ) ) {
	                $attr['class'] .= ' gm-default-featured-img';
	        } else {
	                $size_class = $size;
	                if ( is_array( $size_class ) ) {
	                        $size_class = 'size-' . implode( 'x', $size_class );
	                }$attr = array( 'class' => "attachment-{$size_class} default-featured-img" );
	        }
	        $html = wp_get_attachment_image( $default_thumbnail_id, $size, false, $attr );
	        return $html;
	}