Wordpress custom GD editor with `fill_background` ability to fill PNG alpha with color fill

Custom_WP_Image_Editor_GD.php

<?php
class Custom_WP_Image_Editor_GD extends WP_Image_Editor_GD {
	/**
	 * @param array $rgb
	 *
	 * @return false|resource
	 */
	public function fill_background( $rgb ) {
		$width  = $this->size['width'];
		$height = $this->size['height'];

		$image_background_resource = imagecreatetruecolor( $width, $height );

		$image_fill_arguments = array_merge(
			array( $image_background_resource ),
			apply_filters( 'wpp_image_editor_gd_background_color', $rgb, $this )
		);

		$image_fill_color = call_user_func_array( 'imagecolorallocate', $image_fill_arguments );

		imagefilledrectangle( $image_background_resource, 0, 0, $width, $height, $image_fill_color );
		imagealphablending( $this->image, true );

		$is_copied = imagecopy( $image_background_resource, $this->image, 0, 0, 0, 0, $width, $height );

		if ( $is_copied ) {
			imagedestroy( $this->image );

			$this->image = $image_background_resource;
		} else {
			imagedestroy( $image_background_resource );
		}

		return $is_copied;
	}
}

functions.php

<?php
$output_image_mime_type = 'image/jpeg';
$source_image_path = 'file.png' // output will be `file.png.jpeg`

$wp_image_editors_filter_callback = function () {
  return array( 'Custom_WP_Image_Editor_GD' );
};

add_filter( 'wp_image_editors', $wp_image_editors_filter_callback);

$editor = wp_get_image_editor( $source_image_path );
$editor->fill_background(array(255, 255, 255));
$saved = $editor->save( "{$source_image_path}.$output_image_format", array( 'mime_type' => $output_image_mime_type ) );

remove_filter( 'wp_image_editors', $wp_image_editors_filter_callback);