updated guide about i18n and ICU

docs/guide/tutorial-i18n.md

@@ -497,9 +497,29 @@ See the [data formatter section](output-formatter.md) for details.
 Setting up your PHP environment <a name="setup-environment"></a>
 -------------------------------
 
-TDB
+Yii uses the [PHP intl extension](http://php.net/manual/en/book.intl.php) to provide most of its internationalization features
+such as the number and date formatting of the [[yii\i18n\Formatter]] class and the message formatting using [[yii\i18n\MessageFormatter]].
+Both classes provides a fallback implementation that provides basic functionality in case intl is not installed.
+This fallback implementation however only works well for sites in english language and even there can not provide the
+rich set of features that is avialable with the PHP intl extension, so its installation is highly recommended.
 
-> Note that formatting may differ between different versions of the ICU library compiled with PHP and also based on the fact whether the
-> [PHP intl extension](http://php.net/manual/en/book.intl.php) is installed or not. So to ensure your website works with the same output
-> in all environments it is recommended to install the PHP intl extension in all environments and verify that the version of the ICU library
-> is the same.
+The [PHP intl extension](http://php.net/manual/en/book.intl.php) is based on the [ICU library](http://site.icu-project.org/) which
+provides the knowledge and formatting rules for all the different locales. According to this fact the formatting of dates and numbers
+and also the supported syntax available for message formatting differs between different versions of the ICU library that is compiled with
+you PHP binary.
+
+To ensure your website works with the same output in all environments it is recommended to install the PHP intl extension
+in all environments and verify that the version of the ICU library compiled with PHP is the same.
+
+To find out which version of ICU is used by PHP you can run the following script, which will give you the PHP and ICU version used.
+
+```
+<?php
+echo "PHP: " . PHP_VERSION . "\n";
+echo "ICU: " . INTL_ICU_VERSION . "\n";
+```
+
+We recommend an ICU version greater or equal to version ICU 49 to be able to use all the features described in this document.
+One major feature that is missing in Versions below 49 is the `#` placeholder in plural rules.
+See <http://site.icu-project.org/download> for a list of available ICU versions. Note that the version numbering has changed after the
+4.8 release so that the first digits are now merged: the sequence is ICU 4.8, ICU 49, ICU 50.