minor #1133 Update intro.rst (kix)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #1133).

Discussion
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Update intro.rst

Added PHP opening tag to make things more clear

Commits
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cce42f5c Update intro.rst

doc/intro.rst

@@ -23,6 +23,7 @@ Usage
 
 .. code-block:: php
 
+    <?php
     // web/index.php
     require_once __DIR__.'/../vendor/autoload.php';