PHP側で

echo $_GET['callback'] . '(' . json_encode($object) . ');';

JS側で

$(function(){
   $.getJSON('foo.php?callback=?',
    {},  // No additional parameters sent
    function (data) {
       // data is now JSON object instantiated from retrieved info
       alert(data*'key'*);
   });
});

参考：
Mantis and Jessamine | Creating a JSONP Data Provider in PHP