update sample app intro

Author: lindseymoore

source/read.txt

@@ -43,9 +43,11 @@ On this page, you can see copyable code examples that show common
    provided in each section.
 
 To use an example from this page, copy the code example into the
-:ref:`sample application <php-read-sample>` or your own application.
-Be sure to replace all placeholders, such as ``<connection string>``, with
-the relevant values for your MongoDB deployment.
+:ref:`sample application <php-index-sample>` or your own application.
+Make sure to set the ``MONGODB_URI`` environment variable to the
+connection string for your MongoDB deployment, and replace the
+``<database>`` and ``<collection>`` placeholders with values for your
+target namespace.
 
 .. _php-read-sample: