Fix missing ending statement in php example (#322)

diff --git a/_includes/code/hello.php b/_includes/code/hello.php
index ce75126..b914e6e 100644
--- a/_includes/code/hello.php
+++ b/_includes/code/hello.php
@@ -6,3 +6,4 @@
     echo $greeting;
     return ["greeting" => $greeting];
 }
+?>