This Stack Overflow question about case-insensitive routing with the Slim Framework caught my eye recently. The question asks, in part:

How can I avoid setting up two separate routes [with different cases] that trigger the same callback function?

I found the question intriguing because 1) I love Slim and 2) I’ve never really thought about whether or not URLs are case-sensitive to begin with. My immediate thought was, “They’re already case-insensitive! Has dude even tested this?”

Are URLs Case-sensitive?

Well, kinda. It frequently, but not always, boils down to whether or not the web server’s filesystem is case sensitive. The HTTP server (Apache, nginx, etc) can get involved, as they can (always?) be configured to serve URLs with or without regard to case sensitivity. The web application and/or framework is involved too. If all other factors are case-insensitive but your application’s router is case-sensitive, then your application’s URLs will be too. This is the case with the Slim Framework.

Should URLs Be Case-sensitive?

Again, kinda. In “HTML and URLs”, the w3c has this to say:

URLs in general are case-sensitive (with the exception of machine names). There may be URLs, or parts of URLs, where case doesn’t matter, but identifying these may not be easy. Users should always consider that URLs are case-sensitive.

In general and should are the operative words there. It’s OK either way¹, but assuming case-sensitivity is prudent.

So What About Slim?

URLs in the Slim Framework are case-sensitive². That’s correct based on what we learned about URL case-sensitivity from the w3c, but what if you have a use case that requires case-insensitive URLs? What do do then?

The Magic of Slim Hooks

What are Slim Hooks? From the documentation:

A “hook” is a moment in the Slim application lifecycle at which a priority list of callables assigned to the hook will be invoked. A hook is identified by a string name.

Slim provides six default hooks, three invoked before the current route is dispatched and three invoked after. By using one of the hooks that’s invoked before the route is matched, we can alter the incoming URL’s path to match the case of the routes we’ve defined in our Slim application.

The Case-insensitive Route Hook

First, if there are any routes with mixed case, change them all to lower case. That’s the de facto standard for creating routes anyhow, and if any requests come in that match the old, mixed case routes, we’ll take care of those with the hook in the below example. Same issue, we’re just flipping it on its head. Seriously, don’t get weird about changing those routes.

Next, register this callback on the slim.before.router hook:

1
2
3

$app->hook('slim.before.router', function () use ($app) {
    $app->environment['PATH_INFO'] = strtolower($app->environment['PATH_INFO']);
});

This works because Slim matches the routes you’ve defined against the Slim Environment’s PATH_INFO (originally found in $_SERVER['PATH_INFO']). Since your routes are lower case and the incoming request paths are lower case, you’ve accomplished case insensitive routing in 3 lines of code. BOOM.