Is it possible for a PHP script to insert a block of code into a database, and then some kind of daemon sees that it was put in, and then runs the code? Kind of like a cron job, or a job queue.
Technically you can do it. The code should be stored in a TEXT column and then you could evaluate it with eval: http://es.php.net/manual/en/function.eval.php
This is not a very good idea for several reasons: code stored in a database is not under version control, and the security implications are pretty severe. Most job queue systems store a "job type" and "job parameters" instead of executable code. The job type could be the name of a file to include, or the name of a class to instantiate in an OOP setting. Then you would call a specific function defined there, passing in the parameters.
The job parameters can be any PHP data structure if you use the serialize function to turn it into a string first. http://es.php.net/manual/en/function.serialize.php