Add srcset attribute to Cover Image, based on generated sizes

Ultimate Member change Cover Image HTML

add_filter( 'um_user_cover_photo_html__filter', 'um_cover_image_add_srcset' );

function um_cover_image_add_srcset( $cover_html, $cover_uri, $alt, $is_default, $attrs ) {
	$cover_min_width = UM()->options()->get( 'cover_min_width' );

	$base_dir = realpath( UM()->uploader()->get_upload_base_dir() ) . DIRECTORY_SEPARATOR;
	$base_url = UM()->uploader()->get_upload_base_url();

	//multisite fix for old customers
	if ( is_multisite() ) {
		$multisite_fix_dir = str_replace( DIRECTORY_SEPARATOR . 'sites' . DIRECTORY_SEPARATOR . get_current_blog_id() . DIRECTORY_SEPARATOR, DIRECTORY_SEPARATOR, $base_dir );
		if ( is_dir( $multisite_fix_dir ) ) {
			$base_dir = realpath( $multisite_fix_dir ) . DIRECTORY_SEPARATOR;
			$base_url = str_replace( '/sites/' . get_current_blog_id() . '/', '/', $base_url );
		}
	}

	$user_base_dir = $base_dir . um_user( 'ID' ) . DIRECTORY_SEPARATOR;
	$user_base_url = $base_url . um_user( 'ID' ) . '/';

	$images = array();
	$matchs = array();

	$files = glob( $user_base_dir . 'cover_photo*', GLOB_BRACE );

	foreach ( $files as $file ) {
		$filename = basename( $file );
		preg_match( '/cover_photo\-?(\d+)/', $filename, $matchs );
		$width = isset( $matchs[1] ) ? $matchs[1] : $cover_min_width;
		$images[$width] = "{$user_base_url}{$filename} {$width}w";
	}

	ksort( $images );
	
	$srcset = apply_filters( 'um_user_cover_photo_srcset__filter', implode( ', ', array_unique( $images ) ), um_user( 'ID' ), $files );
	$srcset_attr = ! empty( $srcset ) ? ' srcset="' . esc_attr( $srcset ) . '"' : '';
	
	$cover_html = $cover_uri ? '<img ' . $srcset_attr . ' src="' . esc_attr( $cover_uri ) . '" alt="' . esc_attr( $alt ) . '" />' : '';
	
	return $cover_html;
}