If you decline, you would be making the same mistake some companies do by only hiring developers with previous experience in a very specific set of technologies

If you decline, you would be making the same mistake some companies do by only hiring developers with previous experience in a very specific set of technologies. Every developer who works with WordPress/PHP/MySQL, or any other technology, at some point didn't have experience with them. Everyone has to start somewhere or there would be no developers at all. Whether you disclose your lack of experience to the client depends on the nature of the job. If they only need a basic installation with some plugins and theme customization, take it. Give a reasonable time frame for delivery and deliver it. If on the other hand, they need you to scale their multi-million page views network of blogs, or develop a mission-critical super-complex plugin in a short time frame, then you might want to tell them you haven't done this before, but are willing to work with them to help them achieve their goals.