Is there somewhere in the PHP Manual that tells where it is appropriate to use single quotes and where it is appropriate to use double quotes with the predefined variables?
$var = 'hello';
echo "Say $var"; //works - PHP expands the variable inside double quotes
echo "Say " . $var; // works
echo 'Say $var'; // does not work
echo 'Say ' . $var ; // works
$_POST['msg'] = "hello"; // $_POST is an array
echo "Say $_POST['msg']"; // does not work - does NOT expand the var in an array
echo "Say " . $_POST['msg']; // works
echo 'Say $_POST['msg']'; // does not work
echo 'Say ' . $_POST['msg'] ; // works