A render function for a WordPress Gutenberg block in PHP

wp-plugin-gutenberg-block-render.php

<?php
    /**
     * CALLBACK
     * 
     * Render callback for the dynamic block.
     * 
     * Instead of rendering from the block's save(), this callback will render the front-end
     *
     * @since    1.0.0
     * @param $att Attributes from the JS block
     * @return string Rendered HTML
     */
    public function block_dynamic_render_cb ( $att ) {

        // Coming from RichText, each line is an array's element
        $sum = $att['number1'][0] + $att['number2'][0]; 

        $html = "<h1>$sum</h1>";

        return $html;

    }