Fix E_WARNING in CI_Security::entity_decode() on PHP<5.3.4

Related: #3057
Previous commit: 487d1ae060e6414e0a59c9752a4914fa3b8c4710
diff --git a/system/core/Security.php b/system/core/Security.php
index 8ad0518..6b11405 100755
--- a/system/core/Security.php
+++ b/system/core/Security.php
@@ -644,7 +644,12 @@
 			{
 				if ( ! isset($_entities))
 				{
-					$_entities = array_map('strtolower', get_html_translation_table(HTML_ENTITIES, $flag, $charset));
+					$_entities = array_map(
+						'strtolower',
+						is_php('5.3.4')
+							? get_html_translation_table(HTML_ENTITIES, $flag, $charset)
+							: get_html_translation_table(HTML_ENTITIES, $flag)
+					);
 
 					// If we're not on PHP 5.4+, add the possibly dangerous HTML 5
 					// entities to the array manually