WordPress/wp-includes/blocks/button.php

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

/**
 * Renders the `core/button` block on the server,
 *
 * @since 6.6.0
 *
 * @param array    $attributes The block attributes.
 * @param string   $content    The block content.
 * @param WP_Block $block      The block object.
 *
 * @return string The block content.
 */
function render_block_core_button( $attributes, $content ) {
	$p = new WP_HTML_Tag_Processor( $content );

	/*
	 * The button block can render an `<a>` or `<button>` and also has a
	 * `<div>` wrapper. Find the a or button tag.
	 */
	$tag = null;
	while ( $p->next_tag() ) {
		$tag = $p->get_tag();
		if ( 'A' === $tag || 'BUTTON' === $tag ) {
			break;
		}
	}

	/*
	 * If this happens, the likelihood is there's no block content,
	 * or the block has been modified by a plugin.
	 */
	if ( null === $tag ) {
		return $content;
	}

	// If the next token is the closing tag, the button is empty.
	$is_empty = true;
	while ( $p->next_token() && $tag !== $p->get_token_name() && $is_empty ) {
		if ( '#comment' !== $p->get_token_type() ) {
			/**
			 * Anything else implies this is not empty.
			 * This might include any text content (including a space),
			 * inline images or other HTML.
			 */
			$is_empty = false;
		}
	}

	/*
	 * When there's no text, render nothing for the block.
	 * See https://github.com/WordPress/gutenberg/issues/17221 for the
	 * reasoning behind this.
	 */
	if ( $is_empty ) {
		return '';
	}

	return $content;
}

/**
 * Registers the `core/button` block on server.
 *
 * @since 6.6.0
 */
function register_block_core_button() {
	register_block_type_from_metadata(
		__DIR__ . '/button',
		array(
			'render_callback' => 'render_block_core_button',
		)
	);
}
add_action( 'init', 'register_block_core_button' );