Small fixes (PR #3022)

- Fix incorrect variable in mb_strlen() compat
- Micro-optimization of array_replace(), array_replace_recursive() compat
diff --git a/system/core/compat/mbstring.php b/system/core/compat/mbstring.php
index 91ea801..314dbe9 100644
--- a/system/core/compat/mbstring.php
+++ b/system/core/compat/mbstring.php
@@ -63,7 +63,7 @@
 	{
 		if (ICONV_ENABLED === TRUE)
 		{
-			return iconv_strlen($str, isset($charset) ? $charset : config_item('charset'));
+			return iconv_strlen($str, isset($encoding) ? $encoding : config_item('charset'));
 		}
 
 		log_message('debug', 'Compatibility (mbstring): iconv_strlen() is not available, falling back to strlen().');