Apply "visibility_required" CS rule to constants

php-cs-fixer fix --rules='{"visibility_required": ["property", "method", "const"]}'

Author: nicolas-grekas

PropertyPath.php

@@ -25,7 +25,7 @@ class PropertyPath implements \IteratorAggregate, PropertyPathInterface
     /**
      * Character used for separating between plural and singular of an element.
      */
-    const SINGULAR_SEPARATOR = '|';
+    public const SINGULAR_SEPARATOR = '|';
 
     /**
      * The elements of the property path.