Fixed the code snippets for the expression language functions

Author: stof

Diff for: components/expression_language/extending.rst

@@ -35,7 +35,7 @@ This method has 3 arguments:
 
     $language = new ExpressionLanguage();
     $language->register('lowercase', function ($str) {
-        is_string(%1$s) ? strtolower(%1$s) : %1$s;
+        return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
     }, function ($arguments, $str) {
         if (!is_string($str)) {
             return $str;
@@ -69,11 +69,7 @@ Override ``registerFunctions`` to add your own functions::
             parent::registerFunctions(); // do not forget to also register core functions
 
             $this->register('lowercase', function ($str) {
-                if (!is_string($str)) {
-                    return $str;
-                }
-
-                return sprintf('strtolower(%s)', $str);
+                return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
             }, function ($arguments, $str) {
                 if (!is_string($str)) {
                     return $str;