diff --git src/wp-admin/includes/post.php src/wp-admin/includes/post.php
index 8db8861c42..47a5257bef 100644
--- src/wp-admin/includes/post.php
+++ src/wp-admin/includes/post.php
@@ -2416,7 +2416,6 @@ function the_block_editor_meta_box_post_form_hidden_fields( $post ) {
 	$classic_output = ob_get_clean();
 
 	$classic_elements = wp_html_split( $classic_output );
-	$hidden_inputs    = '';
 	foreach ( $classic_elements as $element ) {
 		if ( 0 !== strpos( $element, '<input ' ) ) {
 			continue;