Why is it important to understand how PHP matches a catch block?

PHP allows you to have multiple catch blocks. The manual says that when an exception occurs PHP will try to find the first matching exception block, but it isn't entirely clear on exactly how PHP matches "catch" blocks when handling exceptions.

Why is it important to understand how PHP matches a catch block?

It really relates to the fact that you are able to extend the base "Exception" class in PHP to make your own classes. Throwing exceptions that are specific is good practice because it allows you to write catch blocks that are focused on solving just one sort of problem at a time. The SPL exceptions are a good example of hierarchical exception inheritance in case you're wanting to see some practical usage.

In PHP a catch block is a match for an exception if the exception class is either the same as or inherits from the class in the catch clause.

Here's a trivial example:

In this example I've set up an exception class that extends the base exception class. I'm throwing an instance of this child exception. If I didn't know that PHP will match it's parent class I would expect the message "Child exception" to be output.

In actual fact if you run this code (here) you will see that the message "Parent exception" is output because PHP has matched MyException with its parent class.

The PHP manual doesn't make this explicit or obvious when discussing catch blocks or exception inheritance but it's one of those things that's always handy to know.

This implies that when we're coding multiple catch blocks we should place the more general parent classes at the bottom of the list. This prevents them from catching exceptions that are meant for their children.

I discuss this and other gotchas in my study guide for the Zend certification exam.

Comments

Separating business logic from persistence layer in Laravel

There are several reasons to separate business logic from your persistence layer. Perhaps the biggest advantage is that the parts of your application which are unique are not coupled to how data are persisted. This makes the code easier to port and maintain. I'm going to use Doctrine to replace the Eloquent ORM in Laravel. A thorough comparison of the patterns is available here . By using Doctrine I am also hoping to mitigate the risk of a major version upgrade on the underlying framework. It can be expected for the ORM to change between major versions of a framework and upgrading to a new release can be quite costly. Another advantage to this approach is to limit the access that objects have to the database. Unless a developer is aware of the business rules in place on an Eloquent model there is a chance they will mistakenly ignore them by calling the ActiveRecord save method directly. I'm not implementing the repository pattern in all its ...

Using Azure Active directory as an OAuth2 provider for Django

Azure Active Directory is a great product and is invaluable in the enterprise space. In this article we'll be setting it up to provide tokens for the OAuth2 client credentials grant. This authorization flow is useful when you want to authorize server-to-server communication that might not be on behalf of a user. This diagram, by Microsoft, shows the client credentials grant flow. From Microsoft documentation The flow goes like this: The client sends a request to Azure AD for a token Azure AD verifies the attached authentication information and issues an access token The client calls the API with the access token. The API server is able to verify the validity of the token and therefore the identity of the client. The API responds to the client Setting up Azure AD as an OAuth2 identity provider The first step is to create applications in your AD for both your API server and the client. You can find step-by-step instructions on how to register the applications o...

"Word of the Day" PHP script (with word list)

I was looking around for a way to generate a word of the day on the web and didn't find anything. So I coded a quick and dirty script to do it. Just in case anybody does a Google search and manages to find my blog: here is my Word of the Day PHP script : Copy this code snippet into a wordoftheday.php file: $file = fopen("interesting_words.txt","r"); $raw_string = fread($file,filesize("interesting_words.txt")); fclose($file); $words_array = explode("|",$raw_string); echo $words_array[array_rand($words_array)]; Of course the real issue I had was finding a list of interesting words in the right format. Here is the list of interesting words that I used: Copy this into a file called interesting_words.txt : ubiquitous : being or seeming to be everywhere at the same time; omnipresent| ecdysiast : a striptease artist| eleemosynary : of, relating to, or dependent on charity| gregious : c...

Adventures in IT

Search This Blog