Open Source software is a business model, which can work. But there is no guarantee on that.
You could build up your business around your CMS, like:
- offering hosted CMS solutions
- offering support with installations
- provide customization service
- provide import/export service
- write a book on it and sell the pdf as docs
- provide closed source modules (advanced feature xy for only 10$ or so)
But be careful - if choose the last option, you might need to take care on the licensing.
There are two common license types around: copyleft and non-copyleft.
Copyleft is for example GPL. The approach is, if somebody writes something on top of your software, he needs to publish the change as open source too.
THe other on is for example Apache Licsense (AL 2.0). You write the code, and everybody else can do whatever he wants with it. From own experience, you'll get tons of contributions back, even when people are not forced. Benefit: it is attracting business users more, because they can build a new product upon your softwares back.
Both software licenses require to leave the header intact (your name is there, for ever).
Both software licenses exclude any guarantees - use on own risk.
Personally I love open source and I am an active open source coder myself (apache license esp). I have seen business models like that working very well.
You should consider one thing finally: you said, it is PHP. If you deliver your PHP script to a customer, he already has insight into the code. Even when it is not free, he sees the source code and could do nasty things with it. For example, copy it to another host without buying a new license. You could obfuscate, but, well customizations on customer side are not so good then.
If you ask me, do something from the list above. Host it on github, googlecode or try to get it into the apache.org foundation and you will probably get a lots of contributions which makes your software wellknown. Hopefully people buy some of your services. If they don't do you don't want to continue you have at least some great code to show your new employer.