in your PHP file, when you echo your data use json_encode (http://php.net/manual/en/function.json-encode.php)
e.g.
<?php
//plum or data...
$output = array("data","plum");
echo json_encode($output);
?>
in your javascript code, when your ajax completes the json encoded response data can be turned into an js array like this:
$.ajax({
type: "POST",
url: "process.php",
data: somedata;
success function(json_data){
var data_array = $.parseJSON(json_data);
//access your data like this:
var plum_or_whatever = data_array['output'];.
//continue from here...
}
});