diff --git a/lib/block-supports/index.php b/lib/block-supports/index.php
index 599ef9276acc69..3b0f0d1540771f 100644
--- a/lib/block-supports/index.php
+++ b/lib/block-supports/index.php
@@ -104,12 +104,15 @@ function gutenberg_apply_block_supports( $block_content, $block ) {
$block_root->setAttribute( 'style', implode( '; ', $new_styles ) . ';' );
}
- $result = '';
- // phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
- foreach ( $body_element->childNodes as $child_node ) {
- $result .= $dom->saveHtml( $child_node );
- }
- return $result;
+ // Avoid using `$dom->saveHtml( $node )` because the node results may not produce consistent
+ // whitespace for PHP < 7.3. Saving the root HTML `$dom->saveHtml()` prevents this behavior.
+ $full_html = $dom->saveHtml();
+
+ // Find the open/close tags. The open tag needs to be adjusted so we get inside the tag
+ // and not the tag itself.
+ $start = strpos( $full_html, '', 0 ) + strlen( '' );
+ $end = strpos( $full_html, '', $start );
+ return trim( substr( $full_html, $start, $end - $start ) );
}
add_filter( 'render_block', 'gutenberg_apply_block_supports', 10, 2 );
diff --git a/phpunit/class-block-supported-styles-test.php b/phpunit/class-block-supported-styles-test.php
index aebd7c20060933..fef5e2bf45d1a8 100644
--- a/phpunit/class-block-supported-styles-test.php
+++ b/phpunit/class-block-supported-styles-test.php
@@ -880,6 +880,20 @@ public function test_render_block_includes_appended_html() {
$this->assertEquals( 'Hello from the block content!
Appended
', $result );
}
+ /**
+ * Ensure that HTML is correctly extracted with multibyte contents.
+ */
+ public function test_render_block_mb_html() {
+ $this->register_block_type(
+ 'core/example',
+ array( 'render_callback' => true )
+ );
+
+ $result = do_blocks( '' );
+
+ $this->assertEquals( '', $result );
+ }
+
/**
* Should not error when the rendered block is text only.
*/