WordPress/wp-includes/blocks/post-featured-image.php

<?php
/**
 * Server-side rendering of the `core/post-featured-image` block.
 *
 * @package WordPress
 */

/**
 * Renders the `core/post-featured-image` block on the server.
 *
 * @param array    $attributes Block attributes.
 * @param string   $content    Block default content.
 * @param WP_Block $block      Block instance.
 * @return string Returns the featured image for the current post.
 */
function render_block_core_post_featured_image( $attributes, $content, $block ) {
	if ( ! isset( $block->context['postId'] ) ) {
		return '';
	}
	$post_ID = $block->context['postId'];

	$size_slug      = isset( $attributes['sizeSlug'] ) ? $attributes['sizeSlug'] : 'post-thumbnail';
	$post_title     = trim( strip_tags( get_the_title( $post_ID ) ) );
	$featured_image = get_the_post_thumbnail( $post_ID, $size_slug, array( 'alt' => $post_title ) );
	if ( ! $featured_image ) {
		return '';
	}
	$wrapper_attributes = get_block_wrapper_attributes();
	if ( isset( $attributes['isLink'] ) && $attributes['isLink'] ) {
		$featured_image = sprintf( '<a href="%1s">%2s</a>', get_the_permalink( $post_ID ), $featured_image );
	}

	$has_width  = ! empty( $attributes['width'] );
	$has_height = ! empty( $attributes['height'] );
	if ( ! $has_height && ! $has_width ) {
		return "<figure $wrapper_attributes>$featured_image</figure>";
	}

	if ( $has_width ) {
		$wrapper_attributes = get_block_wrapper_attributes( array( 'style' => "width:{$attributes['width']};" ) );
	}

	if ( $has_height ) {
		$image_styles = "height:{$attributes['height']};";
		if ( ! empty( $attributes['scale'] ) ) {
			$image_styles .= "object-fit:{$attributes['scale']};";
		}
		$featured_image = str_replace( 'src=', 'style="' . esc_attr( $image_styles ) . '" src=', $featured_image );
	}

	return "<figure $wrapper_attributes>$featured_image</figure>";
}

/**
 * Registers the `core/post-featured-image` block on the server.
 */
function register_block_core_post_featured_image() {
	register_block_type_from_metadata(
		__DIR__ . '/post-featured-image',
		array(
			'render_callback' => 'render_block_core_post_featured_image',
		)
	);
}
add_action( 'init', 'register_block_core_post_featured_image' );