Should I stick with Sun's Java code conventions for PHP code?
For PHP, i'd suggest to follow Zends suggestions
As you might know, Zend is the most widely used framework!
If you are in a business follow the business code convention.
If it's for a personal project you can get the specific language specification (if you do Java than Java, if you do Php than PHP). If it's your personal project you can change few things if you desire...
If you do open source project, you should go see what's already in place.
There are many different coding conventions out there. Have a look at what other people use (read some example code and see how easy it is to understand what is being done) and take your pick.
The important part is to choose one and stick to it.
You have a number of options:
Zend: http://framework.zend.com/manual/en/coding-standard.html
Pear: http://pear.php.net/manual/en/standards.php
Wordpress: http://codex.wordpress.org/WordPress_Coding_Standards
But like prakash suggests, Zend is a good choice.