I want to build a generic rule parser for pen and paper style RPG systems. A rule can involve usually 1 to N entities 1 to N roles of a dice and calculating values based on multiple attributes of an entity.

For example:

Player has STR 18, his currently equipped weapon gives him a bonus of +1 STR but a malus of DEX -1. He attacks a monster entity and the game logic now is required to run a set of rules or actions:

Player rolls the dice, if he gets for example 8 or more (base attack value he needs to pass is one of his base attributes!) his attack is successfully. The monster then rolls the dice to calculate if the attack goes through it's armor. If yes the damage is taken if not the attack was blocked.

Besides simple math rules can also have constraints like applying only to a certain class of user (warrior vs wizzard for example) or any other attribute. So this is not just limited to mathematical operations.

If you're familiar with RPG systems like Dungeon and Dragons you'll know what I'm up to.

My issue is now that I have no clue how to exactly build this the best possible way. I want people to be able to set up any kind of rule and later simply do an action like selecting a player and a monster and run an action (set of rules like an attack).

I'm asking less for help with the database side of things but more about how to come up with a structure and a parser for it to keep my rules flexible. The language of choice for this is php by the way.