Arnold’s wor(l)ds

I’ve enabled pretty URLs in Wordpress. It appears that has caused all articles to reappear on planet PHP with today as date. This was not my intention.

Only ‘How I PHP: Exceptions with Interfaces’ is a new article, the others are old ones.

There are some techniques that I use in almost all of my projects, but I hardly see in other projects. To my opinion others are missing out, so I’ll discuss a few of them. This time I want to talk about exceptions. If you are already using exceptions, the first 2 paragraph won’t be anything new to you, but the third one might.

Why you should use exceptions

I often see something like this:

$db = new mysqli('localhost', 'user', 'pwd', 'mydb');
...
function activateCustomer($id) {
  $result = $db->query("UPDATE customer SET status='ACTIVE' WHERE id = " . (int)$id) or die("Query failed: " . $db->error);
  ...
}

I know that an example like this even made it into the PHP documentation, but I don’t like it at all. When an error occurs, you want the error handler to pick it up. The handler might simply display the error, which is fine for the test environment, but in a live environment it should mail you the error and display a ‘Sorry, unexpected error’ message to the user. This is not possible when using die().

Instead you can use trigger_error($msg, E_USER_ERROR). However this will still end the execution of the script, while the failure might cause an action to while, while the page can still be displayed. We can solve this nicer with an exception.

function activateCustomer($id) {
  $result = $db->query("UPDATE customer SET status='ACTIVE' WHERE id = " . (int)$id);
  if (!$result) throw new Exception("Query failed: " . $db->error);
  ...
}

If the exceptions isn’t caught, it will just produce a fatal error. However, in this example, only the action fails on a query error, the page can probably be rendered. So we can catch the exception and display a ‘Sorry, action failed’ message.

try {
  activateCustomer($_GET['id']);
} catch (Exception $e) {
  mailExceptionToSiteadmin($e);  # a user function that mails the error info to you
  echo "Sorry, an unexpected error occurred when activating the customer";
}
...

Catching specific exceptions

In the previous example I threw and caught a general Exception. However in some cases you only want to catch certain exceptions. Have a look at the following example.

class Item_Exception extends Exception {}
class ItemNotFound_Exception extends Item_Exception {}
 
try {
  ...
  if (!isset($items[$_GET['id'])) throw new ItemNotFound_Exception("Could not find item '{$_GET['id']}'");
} catch (Item_Exception $e) {
  echo "This item can't be used: " . $e->getMessage();
}
...

Using interface to specify exceptions

Until so far, nothing new here. The examples above cover 90% of all the situations. However at times, you need to specify than an exception can be cached in certain occasions while it is already extending a class. To solve this we can use an interface. PHP actually uses something like ‘instanceof’ with the catch, so instead of using a class name, we can also use an interface.

interface InputFault {}
class Item_Exception extends Exception {}
class ItemNotFound_Exception extends Item_Exception implements InputFault {}
 
try {
  ...
  if (!isset($items[$_GET['id'])) throw new ItemNotFound_Exception("Could not find item '{$_GET['id']}'");
} catch (InputFault $e) {
  echo "Invalid data was entered: " . $e->getMessage();
} catch (Exception $e) {
  mailExceptionToSiteadmin($e);  # a user function that mails the error info to you
  echo "Sorry, Something went wrong";
}
...

Today I’m very pleased. My UDF lib ‘lib_mysqludf_xql’ has been mentioned in an article by Jon Stephens about ‘Using XML in MySQL 5.1 and 6.0′.

The lib_mysqludf_xql you can query relational data and return XML. It will give you the same functionality as SQL/XML in Oracle and MS SQL. Compares to other methods like using CONCAT or generating XML in a PHP script, this method is much easier and will perform much better.

You can find the library at mysqludf.org, including some documentation. You can also find a bunch of other nice libs there.

Do note that you need to have root privileges on the server to install MySQL UDFs.

Last time, after write the ‘How I PHP: The Output Handler’ article, David Arnold asked a question about adding some dynamic data in your template. For instance, it would be nice if the data in the left column, ‘Beauty Tips’, could be different for each page. There are several ways to solve this, but for know I will choose the simplest.

We’ll start with making changing template file, adding markers for the title and for the left menu.
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