Without seeing your complete code, it’s a bit difficult to answer. Also it’s a big vague when you say that it’s not calling the function (are you getting any errors / warnings?) If you’ve got error reporting turned off, please turn it on.
Also, try this:
require_once('reviews.php');
reviewsPanel();
If the file is not included, “require” will throw a Fatal Error.
One other thing I see is that the echo “test” in the reviewPanel() function is outside the PHP tags. So that will not be executed as PHP code.