[index.php] Add extension check for 'mbstring'

The mbstring extension is required by all formats in order to convert multi-
byte characters to UTF-8. This commit adds an extension check to throw an
error message if the extension is not enabled.

index.php

@@ -70,6 +70,9 @@
 if(!extension_loaded('libxml'))
 	die('"libxml" extension not loaded. Please check "php.ini"');
 
+if(!extension_loaded('mbstring'))
+	die('"mbstring" extension not loaded. Please check "php.ini"');
+
 // configuration checks
 if(ini_get('allow_url_fopen') !== "1")
 	die('"allow_url_fopen" is not set to "1". Please check "php.ini');