Cast to array because this could be empty; update requirements description

INSTALL

@@ -2,17 +2,20 @@
 REQUIREMENTS
 ============
 
-* The Apache Webserver
+* The Apache or Lighttpd Webserver
 * .htaccess support allowing overrides for DirectoryIndex
-* PHP Version 4.3.1 or greater
-* PCRE (perl compatible regular expression) installed with PHP
+* PHP Version 4.3.1 or greater including
+   - PCRE (perl compatible regular expression)
+   - libiconv (recommended)
+   - mbstring (optional)
 * php.ini options:
    - error_reporting E_ALL & ~E_NOTICE (or lower)
    - file_uploads on (for attachment upload features)
    - memory_limit (increase as suitable to support large attachments)
 * PHP compiled with Open SSL to connect to IMAPS and to use the spell checker
 * A MySQL or PostgreSQL database engine or the SQLite extension for PHP
-* A database with permission to create tables
+* One of the above databases with permission to create tables
+* An SMTP server or PHP configured for mail delivery
 
 
 INSTALLATION

program/include/rcube_imap.inc

@@ -2494,7 +2494,7 @@ class rcube_imap
     if (!$mbox_name)
       $mbox_name = $this->mailbox;
       
-    $index = array_flip($this->uid_id_map[$mbox_name]);
+    $index = array_flip((array)$this->uid_id_map[$mbox_name]);
     if (isset($index[$id]))
       $uid = $index[$id];
     else