You didn't provide enough specs to provide much help on.
First problem is to look into how you'll get the data off the PDA and to your web server. Remember: PHP code does not run on the client machine nor the client's PDA.
The next battle is to convert the PDA's data format to the PHP calendar and then import/sync it.
The sync problem is another one. What if Event X is is both in the PDA and the web site and both get changed. Which one gets the final copy? How will you get the data back to the PDA?
I'm pretty sure you can find stuff out there to convert Palm's data format. I had built a script to convert Palm's address book format and import it into a MySQL database. Its not hard to work with. Just requires some work to figure things out.