Default value need to have PHP tags surrounded

js/php.admin.js

@@ -26,7 +26,7 @@
 
       $('[data-drupal-selector="edit-visibility-php"]').drupalSetSummary(function (context) {
         var $code = $(context).find('textarea[name="visibility[php][php]"]');
-        if ($code.val() === 'return TRUE;') {
+        if ($code.val() === '<?php return TRUE; ?>') {
           return Drupal.t('Not restricted');
         }
         else {

src/Plugin/Condition/Php.php

@@ -27,7 +27,7 @@ class Php extends ConditionPluginBase {
     // By default the PHP snippet need to return TRUE or blocks will silently
     // disappear after the module has been enabled and/or a block has been
     // configured without configuring a PHP snippet.
-    return ['php' => 'return TRUE;'] + parent::defaultConfiguration();
+    return ['php' => '<?php return TRUE; ?>'] + parent::defaultConfiguration();
   }
 
   /**