Add notes about new requirements of the new mysql database driver,

add check in installer (#1488875)

INSTALL

@@ -14,6 +14,7 @@ REQUIREMENTS
 * PHP Version 5.2.1 or greater including
    - PCRE, DOM, JSON, XML, Session, Sockets (required)
    - PHP Data Objects (PDO) with driver for either MySQL, PostgreSQL or SQLite (required)
+     Note: MySQL database driver requires PHP 5.3 or newer.
    - Libiconv, Zip (recommended)
    - Fileinfo, Mcrypt, mbstring (optional)
 * PEAR packages distributed with Roundcube or external:

UPGRADING

@@ -20,7 +20,7 @@ target location and check and update the configuration and database schema.
 After all is done, the temporary folder with the new Roundcube files can be 
 removed again.
 
-Please also see Post-Upgrade Activities section.
+WARNING: See Post-Upgrade Activities section below.
 
 
 Updating manually
@@ -53,3 +53,4 @@ Post-Upgrade Activities
 3. When upgrading from version older than 0.6-beta you should make sure
    your folder settings contain namespace prefix. For example Courier users
    should add INBOX. prefix to folder names in main configuration file.
+4. Check system requirements in INSTALL file.

installer/check.php

@@ -130,7 +130,13 @@ foreach ($optional_php_exts as $name => $ext) {
 $prefix = (PHP_SHLIB_SUFFIX === 'dll') ? 'php_' : '';
 foreach ($RCI->supported_dbs as $database => $ext) {
     if (extension_loaded($ext)) {
-        $RCI->pass($database);
+        // MySQL driver requires PHP >= 5.3 (#1488875)
+        if ($ext == 'pdo_mysql' && version_compare(PHP_VERSION, '5.3.0', '<')) {
+            $RCI->fail($database, 'PHP >= 5.3 required');
+        }
+        else {
+            $RCI->pass($database);
+        }
     }
     else {
         $_ext = $ext_dir . '/' . $prefix . $ext . '.' . PHP_SHLIB_SUFFIX;