Just off the top of my head:
Good PHP code should be consistent. Whether that means setting rules for the names of variables and functions, adopting standard approaches to recurring tasks like database access and error handling, or simply making sure all of your code is indented the same way, consistency makes your code easier for others to read.
Good PHP code should be portable. PHP has a number of features, such as magic quotes and short tags, that can break fragile code when they are switched on or off. If you know what you're doing, however, you can write code that works by adapting to its environment.
Good PHP code should be secure. While PHP offers excellent performance and flexibility out of the box, it leaves important issues like security entirely in the hands of the developer. A deep understanding of potential security holes like Cross-Site Scripting (XSS), Cross-Site Request Forgeries (CSRF), code injection vulnerabilities, and character encoding loopholes is essential for a professional PHP developer these days.