null,
				'color' => isset( $border['color'] ) && ! wp_should_skip_block_supports_serialization( $block_type, '__experimentalBorder', 'color' ) ? $border['color'] : null,
				'style' => isset( $border['style'] ) && ! wp_should_skip_block_supports_serialization( $block_type, '__experimentalBorder', 'style' ) ? $border['style'] : null,
			);
			$border_block_styles[ $side ] = $border_side_values;
		}
	}

	// Collect classes and styles.
	$attributes = array();
	$styles     = wp_style_engine_get_styles( array( 'border' => $border_block_styles ) );

	if ( ! empty( $styles['classnames'] ) ) {
		$attributes['class'] = $styles['classnames'];
	}

	if ( ! empty( $styles['css'] ) ) {
		$attributes['style'] = $styles['css'];
	}

	return $attributes;
}

/**
 * Checks whether the current block type supports the border feature requested.
 *
 * If the `__experimentalBorder` support flag is a boolean `true` all border
 * support features are available. Otherwise, the specific feature's support
 * flag nested under `experimentalBorder` must be enabled for the feature
 * to be opted into.
 *
 * @since 5.8.0
 * @access private
 *
 * @param WP_Block_Type $block_type    Block type to check for support.
 * @param string        $feature       Name of the feature to check support for.
 * @param mixed         $default_value Fallback value for feature support, defaults to false.
 * @return bool Whether the feature is supported.
 */
function wp_has_border_feature_support( $block_type, $feature, $default_value = false ) {
	// Check if all border support features have been opted into via `"__experimentalBorder": true`.
	if ( $block_type instanceof WP_Block_Type ) {
		$block_type_supports_border = isset( $block_type->supports['__experimentalBorder'] )
			? $block_type->supports['__experimentalBorder']
			: $default_value;
		if ( true === $block_type_supports_border ) {
			return true;
		}
	}

	// Check if the specific feature has been opted into individually
	// via nested flag under `__experimentalBorder`.
	return block_has_support( $block_type, array( '__experimentalBorder', $feature ), $default_value );
}

// Register the block support.
WP_Block_Supports::get_instance()->register(
	'border',
	array(
		'register_attribute' => 'wp_register_border_support',
		'apply'              => 'wp_apply_border_support',
	)
);
<br />
<b>Fatal error</b>:  Uncaught Error: Failed opening required '/home/valigeria/public_html/wp-includes/vars.php' (include_path='.:/opt/alt/php81/usr/share/pear:/opt/alt/php81/usr/share/php:/usr/share/pear:/usr/share/php') in /home/valigeria/public_html/wp-settings.php:495
Stack trace:
#0 /home/valigeria/public_html/wp-config.php(111): require_once()
#1 /home/valigeria/public_html/wp-load.php(50): require_once('/home/valigeria...')
#2 /home/valigeria/public_html/wp-blog-header.php(13): require_once('/home/valigeria...')
#3 /home/valigeria/public_html/index.php(17): require('/home/valigeria...')
#4 {main}
  thrown in <b>/home/valigeria/public_html/wp-settings.php</b> on line <b>495</b><br />