php $_GET and undefined index

Error reporting will have not included notices on the previous server which is why you haven't seen the s://forum.tuteehub.com/tag/error">errors.

You should be checking whether the index s actually exists in the $_GET array before attempting to use it.

Something like this would be suffice:

if (isset($_GET['s'])) {
    if ($_GET['s'] == 'jwshxnsyllabus')
        echo "syllabus', '../syllabi/jwshxnporsyllabus.xml',         '../bibliographies/jwshxnbibliography_')\">";
    else if ($_GET['s'] == 'aquinas')
        echo "syllabus', '../syllabi/AquinasSyllabus.xml')\">"; 
    else if ($_GET['s'] == 'POP2')
        echo "";
} else {
    echo "";
}

It may be beneficial (if you plan on adding more cases) to use a switch statement to make your code more readable.

switch ((isset($_GET['s']) ? $_GET['s'] : '')) {
    case 'jwshxnsyllabus':
        echo "syllabus', '../syllabi/jwshxnporsyllabus.xml',         '../bibliographies/jwshxnbibliography_')\">";
        break;
    case 'aquinas':
        echo "syllabus', '../syllabi/AquinasSyllabus.xml')\">";
        break;
    case 'POP2':
        echo "";
        break;
    default:
        echo "";
        break;
}

EDIT: BTW, the first set of code I wrote mimics what yours is meant to do in it's entirety. Is the expected outcome of an unexpected value in ?s= meant to output no  tag or was this an oversight? Note that the switch will fix this by always defaulting to .