PHP Timezone Script

I have a need for members to choose the time zone that they are in. They will choose from a drop down list. I would like the list to be extensive as possible without being too rediculous. The list should be organized by continents. ## Deliverables The timezone will be selected on a registration page using a drop down selection box in a web form. I am looking for a list of cities, organized by continents, that people would recognize as being in the same time zone that they are in. When selected, I will save their selection as a number in a MySQL members table. That number must determine how much time ahead or behind they are compared to the server's timezone. This info will be used in displaying the proper time in? comments, posts, or any entries that a member? views on my website. All data entered in posts will be saved using servers time. I will convert to members timezone when displaying information to them. I need a PHP script that will convert server times to current members time. Your work must be your own so that I can legally own it and include it in a software package for resell if I choose to. Let me know if this is not clear.