Fix implicitly nullable $callback parameter

Author: mcaskill

Function.HTML-Build-Attributes.php

@@ -14,7 +14,7 @@
      * @return string Returns a string of HTML attributes
      *     or a empty string if $attributes is invalid or empty.
      */
-    function html_build_attributes($attributes, callable $escape = null)
+    function html_build_attributes($attributes, ?callable $escape = null)
     {
         if (is_object($attributes) && !($attributes instanceof \Traversable)) {
             $attributes = get_object_vars($attributes);

README.md

@@ -23,7 +23,7 @@ breaking changes may occur between major releases.
 ## API
 
 ```php
-html_build_attributes( array|object $attr [, callable $callback = null ] ) : string
+html_build_attributes( array|object $attr [, ?callable $callback = null ] ) : string
 ```
 
 ### Parameters