Go to Tools/Options, tap "File Types". Is there an entry "PHP" in the select box? If yes, please verify that the file extension "php" is included. In the Syntax box, select "PHP+HTML". Also select the "PHP-HTML" color scheme.
All of this was allready set right. Opening a file via ftp worked. Later also local files had syntax highlighting. I didn't change a thing. Maybe a reboot did it?