PHP installation check command added

Author: eddiejaoude

php/lesson1/tutorial.md

@@ -7,6 +7,7 @@ title: Introduction to PHP
 
 * 15 minutes
 * PHP installed - this is by default on most Operating Systems. But if not, here are the official [installation instructions](http://php.net/manual/en/install.php)
+    * Check installation by running the following command in cli `php -v`, this should output the version, eg. **5.5.9**
 
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