jQuery getJSON to external PHP page

It could be that you haven't got a callback in test.php. Also, json_encode only accepts an array:

$results = array("key" => "value");
echo $_GET['callback'] . '(' . json_encode($results) . ')';
// the callback stuff is only needed if you're requesting from different domains

jQuery automatically switches to JSONP (i.e. using script tags instead of XMLHttpRequest) when you use http://. If you have test.html and test.php on the same domain, try using relative paths (and no callbacks).

Be careful with moff's answer. It's got a common XSS vulnerability: http://www.metaltoad.com/blog/using-jsonp-safely