jQuery: jSon

<script type="text/javascript">
   $(document).ready(function () {
      
       var query = "SELECT * FROM tbl_users WHERE user_id = 3";   
      
       $.getJSON("query_executor.php", { query: query}, function(data){
          alert("user_name: " + data[0].user_name);
         
       });        
   });
</script>

O

var html = $.ajax({
            type: "GET",
            url: "php_ajax/query_executor.php",
            data: "query=" + query,
            async: false
         }).responseText;
    
    var myObject = eval('(' + html + ')');
    
    alert(var_dump_rck(myObject[0]));

El PHP

<?php
include("db.php");
    
$query = $_REQUEST['query'];
$result = mysql_query($query)  or trigger_error(mysql_error()); 

$jsonArr = array();
while($row = mysql_fetch_object($result))
{    
    $jsonArr[] = $row;
}

echo json_encode($jsonArr);

?>

El Json Generado

[
 {"user_id":"1","user_name":"pepe","user_fecha_nacimiento":"1986-09-01"},
 {"user_id":"2","user_name":"juan","user_fecha_nacimiento":"1988-12-05"},
 {"user_id":"3","user_name":"Ambrosio Romero","user_fecha_nacimiento":"1984-05-12"}
]