Well, PHP can act as a web server itself since version 5.4, but you don't need to use a full blown web server to run a PHP script. In fact, nothing has to run continuously on your PC to execute PHP scripts. You can run a script as scheduled task once a day or even every hour. All you have to do is
download PHP, unzip it to C:\php\, rename php.ini-production to php.ini and you're ready to execute php scripts. Now you can run php scripts by calling C:\php\php-win.exe my_automation_script.php in command prompt or as any other program in a scheduled task.
It is easy to remove songs from RadioDJ's database. Adding new songs is more complicate, because during import RadioDJ analyses audio files to determine optimal start and end cue points by cutting silence. Unfortunately, I have not come across similar solution in PHP which would not involve running a separate command line application.
Another issue is that the import script would have to parse ID3 tags, which can be done in PHP and I could write a decent script which can do it.
In short, it is doable if you are ok with occasional silence at the beginning and end of imported tracks.
Update: I realised how to make RadioDJ import tracks and perform it's magic; The "Load M3U Playlist" action does import all file into database, but there is a drawback - RadioDJ doesn't assign category, subcategory, track type or genre. However, it can be accomplished by smartly combining two scheduled PHP scripts and one RadioDJ event. First PHP script would generate an M3U file by scanning a directory. Minutes later, RadioDJ would load the M3U file and immediately clear playlist. Then second PHP would run and assign category, subcategory and track type to list of imported files. Updating genres using PHP is more complicated task, but it can be done.