Using PHP functions as modifiers now triggers a deprecation notice (#814)

Fixes #813

libs/plugins/modifiercompiler.strlen.php

@@ -0,0 +1,23 @@
+<?php
+
+/**
+ * Smarty plugin
+ *
+ * @package    Smarty
+ * @subpackage PluginsModifierCompiler
+ */
+/**
+ * Smarty strlen modifier plugin
+ * Type:     modifier
+ * Name:     strlen
+ * Purpose:  return the length of the given string
+ *
+ * @link   https://www.smarty.net/docs/en/language.modifier.strlen.tpl strlen (Smarty online manual)
+  *
+ * @param array $params parameters
+ *
+ * @return string with compiled code
+ */
+function smarty_modifiercompiler_strlen($params) {
+    return 'strlen((string) ' . $params[0] . ')';
+}