On the TutsPlus.com site they've posted a tutorial helping you connect your PHP application with Slack allowing for both the sending of content to your Slack channel(s) but also responding to "slash" commands.

If you've been following the development of office communication tools in the past year or so, I'm sure you've heard plenty about Slack. [...] As developers, we are in a good position to jump on the trend and think about ways we can use Slack as a chat-based user interface for our applications.

That's why, in this tutorial, you will learn how to get started with integrating your PHP application to Slack, by making it post messages to a Slack channel and respond to slash commands.

They start with a "bare bones" setup that will get you up and running and sending messages to your Slack instance. Their example takes in a string and sends it along through a custom Slack application. They walk you through the steps to create this application on the Slack side and this example code to make the connection and send the message. The tutorial walks you through all of the provided code and helps you get your OAuth credentials in place to secure the connection.

With this basic functionality in place you can then build on top of it and define "slash" commands that request a URL. Their example "tells a joke" in the channel making the request.