Since the target audience for PHP User Groups is web-minded folk, it's a fairly safe bet to start your promotion online. There are a multitude of site resources, many of them free to use, that will help you promote your user group.
There are a load of event sites that you have at your disposal. All of these are useful for tracking attendee numbers or ensuring you don't go over the capacity of your venue!
Lanyrd.com is a free offering that allows people to “track” an event (i.e. show an interest without committing to attending) or “attend” an event. It integrates well with Twitter, and the company is under the “Eventbrite” company, so it also integrates nicely with Eventbrite events (such as importing Eventbrite events to Lanyrd).
Eventbrite is primarily a registration/ticket selling site, but because they don't charge if you don't charge, it makes it ideal for registration for free events (such as meetups!). Using Eventbrite events, you can get a pretty good idea of the number of attendees, and Eventbrite also allows you to export the attendee emails to services like Mailchimp so you can keep in touch with your attendees. As a rule of thumb, you should estimate that 60% of those registered for a free meetup will actually turn up.
Meetup.com is a much more socially targeted site that allows registrations. Meetup.com allows you to have a gallery, discussion forum, email your members and more. It also allows you to charge your members, should you wish. However, you must pay a small amount to use it. You are also not able to access the email addresses of your members, which means you cannot use third party services to email your members. Your mileage may vary also, Meetup.com seems very popular in some places, especially in the US, but in other places may not be as effective.
It goes without saying that your user group should have a social presence, which regularly interacts with the community. It's well worth setting up accounts with Twitter, LinkedIn, Facebook and other social networks to allow you to converse with the community.
Google Groups & discussion lists
Some meetups like to have a mailing-list, and Google Groups is a great place to host a community forum with very little fuss. Bear in mind that mailing lists aren't as popular with some people, so your mileage may vary.
Having an announcement list that you can send emails to is invaluable, but it's extremely important that you do not spam your list members as this can damage your reputation. Some sites like Mailchimp allow you to monitor your list performance to ensure you're not spamming.
Running a Website
Some meetups don't even have a website! But we are web developers, we should really have one… even if it is a holding page with information about your meetup, it's worth having this sort of web presence as a canonical and authoritative source of information.
IRC channels are great ways of real-time communication that anyone can use. You can set up free IRC channels using Freenode for example. While you're there, make sure to join other IRC channels of communities that you want to be a part of!
Once you've established your online presence, it's undoubtedly worth looking into a bit of “offline” promotion. Here's some suggestions…
(contact them - start with web companies, then find others who may have web teams)
(e.g. magazines, newspapers etc.)