How to work with jQuery AJAX and PHP array return

I have a jquery ajax request like;

$.ajax({
    type: 'POST',
    url: 'processor.php',
    data: 'data1=testdata1&data2=testdata2&data3=testdata3',
    cache: false,
    success: function(result) {
      if(result){
        alert(result);
      }else{
        alert("error");
      }
    }
});

The handler processor.php is set to return an array like;

$array = array("a","b","c","d");
echo $array;

I want to do action in client side based on this. Say if array[0] is 'b', I want to alert "hi". Again if array[2] is 'x', I want to alert "hello", and so on. How can I filter array elements in order to grab their data?