Insert string into specific part of another string

I've got a URL as a string, for example:

http://ift.tt/1Dout16

I'd like to add another subfolder to it with PHP, before hello, so it should look like this:

http://ift.tt/1C784y4

I thought about using explode to separate the URL by slashes, and adding another one before the last one, but I'm pretty sure I over complicate it. Is there an easier way?