Undefined index error

I'm using the superglobal $_POST array to access URL parameters passed to my page. I was getting undefined constant errors in my error_log file, and found that I needed to quote the variables in the array. $_POST['SortBy'] etc. Now, it appears that I've just traded the undefined constant errors for undefined index warnings. Everytime a page hits a page with url parameters, it triggers a warning in the error_log file. What's triggering these undefined index errors? I suppose I could lower the logging level in php.ini, but I'd rather code properly than to hide the problem. Thanks!