The shortest possible example PHP-rendered WordPress block.

block-hi-test.php

<?php
/**
 * Plugin Name: Block — Hi Test
 * Description: The shortest possible example PHP-rendered WordPress block.
 * Author: Paul Clark
 * Author URI: https://pdclark.com
 *
 * @package hi
 */

/**
 * Register: PHP.
 */
add_action(
	'init',
	function() {
		wp_register_script(
			'hi-test',
			add_query_arg( [ 'action' => 'block-hi.js', ], admin_url( 'admin-ajax.php' ) ),
			[ 'wp-blocks', 'wp-element', 'wp-editor' ],
			microtime(),
			true
		);
		register_block_type(
			'hi/test',
			[
				'editor_script'   => 'hi-test',
				'render_callback' => function( $attributes, $content ) {
					ob_start();
					do_action( 'hi/test/render_callback', $attributes, $content );
					return ob_get_clean();
				},
			]
		);
	}
);

/**
 * Render: PHP.
 *
 * @param array  $attributes Optional. Block attributes. Default empty array.
 * @param string $content    Optional. Block content. Default empty string.
 */
add_action(
	'hi/test/render_callback',
	function( $attributes, $content ) {
		?>
		<h1>Hello world.</h1>
		<?php
		$q = new WP_Query(
			[
				'post_type'   => 'post',
				'numberposts' => 3,
			]
		);
		while ( $q->have_posts() ) {
			$q->the_post();
			?>
			<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a><br/>
			<?php
		}
		wp_reset_postdata();
	},
	10,
	2
);

/**
 * Register: JavaScript.
 * 
 * @see https://developer.wordpress.org/block-editor/how-to-guides/block-tutorial/creating-dynamic-blocks/
 */
add_action(
	'wp_ajax_block-hi.js',
	function() {
		header( 'Content-Type: text/javascript' );
		?>
( function ( blocks, element, serverSideRender, blockEditor ) {
	var el = element.createElement,
		registerBlockType = blocks.registerBlockType,
		ServerSideRender = serverSideRender,
		useBlockProps = blockEditor.useBlockProps;

	registerBlockType( 'hi/test', {
		apiVersion: 2,
		title: 'Hi Test',
		icon: 'megaphone',
		category: 'widgets',

		edit: function ( props ) {
			var blockProps = useBlockProps();
			return el(
				'div',
				blockProps,
				el( ServerSideRender, {
					block: 'hi/test',
					attributes: props.attributes,
				} )
			);
		},
	} );
} )(
	window.wp.blocks,
	window.wp.element,
	window.wp.serverSideRender,
	window.wp.blockEditor
);
		<?php
		exit;
	}
);

add_action('init',function(){
  $sn = 'block-test-test';
  $block_meta = [
    'name' => 'test/test',
    'title' => 'Test',
    //"description" => "Shows warning, error or success notices...",
    //'icon' => 'networking',
    //'category' => 'common',
    'editor_script_handles'   => $sn,
    'render_callback' => function($attrs,$content,$block){
      //return "XXX2".print_r(func_get_args(),true);
      //return "XXX2".print_r($attrs,true).print_r($content,true);
      return "TEST2";
    }
  ];
  $ret = register_block_type($block_meta['name'],$block_meta);
  $sd = <<<EOD
    var block = {
      apiVersion: 3,
      edit: function(props) {
        var bn = props.name
        var bp = wp.blockEditor.useBlockProps()
        
        var bp_ssr = {
          block: bn,
          attributes: props.attributes
        }
        var ss_el = wp.element.createElement(wp.serverSideRender,bp_ssr)
        
        var bp_inner = {
          templateLock: 'all',
          template: [
            [ 'core/image', {} ],
            [ 'core/heading', { placeholder: 'Book Title' } ],
            [ 'core/paragraph', { placeholder: 'Summary' } ],
          ],
        }
        var inner_el = wp.element.createElement(wp.blockEditor.InnerBlocks,bp_inner)
        
        var ret = wp.element.createElement('div',bp,ss_el,inner_el)
        return ret
      },
      save: function(props) {
        //return null
        var bp = wp.blockEditor.useBlockProps.save()
        var inner_el = wp.element.createElement(wp.blockEditor.InnerBlocks.Content)
        
        //var ret = wp.element.createElement('div',bp,inner_el)
        ret = inner_el
        return ret
      }
    }
    wp.blocks.registerBlockType('{$block_meta['name']}',block)
  EOD;
  wp_register_script($sn,'',[ 'wp-blocks', 'wp-element', 'wp-editor' ]);
  wp_enqueue_script($sn);
  wp_add_inline_script($sn,$sd);
});