//处理json数组中文问题
function arrayRecursive(&$array, $function, $apply_to_keys_also = false)
{
    static $recursive_counter = 0;
    if (++$recursive_counter > 1000) {
        die('possible deep recursion attack');
    }
    foreach ($array as $key => $value) {
        if (is_array($value)) {
            arrayRecursive($array[$key], $function, $apply_to_keys_also);
        } else {
            $array[$key] = $function($value);
        }
        if ($apply_to_keys_also && is_string($key)) {
            $new_key = $function($key);
            if ($new_key != $key) {
                $array[$new_key] = $array[$key];
                unset($array[$key]);
            }
        }
    }
    $recursive_counter--;
}

//直接返回json数组
function JSON($array) {
    arrayRecursive($array, 'urlencode', true);
    $json = json_encode($array);
    return urldecode($json);
}

//调用
$data = array();
$data['success']="false";
$data['message']="用户名不存在，请重新输入";
        
echo JSON($data);

$.post("ajax.php?mod=member&code=check_user_exist",{username:username},function(data){
            var json=$.parseJSON(data);
            if(json.success){
                alert(json.success);             
            }else{
                alert(json.success);        
            }
        });