Add the class constant rule

wordpress-coding-standards/php.md

@@ -494,6 +494,18 @@ For those, adding `callable`, class and interface name based parameter type decl
 
 Using the `array` keyword in type declarations is **strongly discouraged** for now, as most often, it would be better to use `iterable` to allow for more flexibility in the implementation and that keyword is not yet available for use in WordPress Core until the minimum requirements are raised to PHP 7.1.
 
+### The ::class constant
+
+When using the `::class` constant for class name resolution you should add no space between the `::class` and the preceding class name it applies to. There should be no space between the double colon and the `class` keyword, and the `class` keyword should be in lowercase.
+
+```php
+// Correct.
+add_action( 'action_name', array( My_Class::class, 'method_name' ) );
+
+// Incorrect.
+add_action( 'action_name', array( My_Class :: CLASS, 'method_name' ) );
+```
+
 ## Declare Statements, Namespace, and Import Statements
 
 ### Namespace declarations