How to generate dynamic if statement according to rules in PHP

I have a list of products for which I generate prices according to simple rules I wrote as if statements, something like

if ($product[rarity] == 'Rare') { $price += 2 }
if ($product[foil]) { $price += $price * 0.1 }
if ($product[weight] >= 6 and $product[weight] < 11) { $price += $price * 0.2 }
if ($product[rarity] == 'Mythic' and $price < 0.7 ) { $price = 0.7 }

But because those rules are being changed very often and their amount rises with each day, I decided to store "rules" as JSON. This should also help me write some kind of UI for other users. My example JSON could look like this

{
    "rules": [{
            "name": "Add 2 to price if rarity is Rare",
            "predicates": [{
                    "field": "rarity",
                    "operator": "==",
                    "value": "Rare"
                }
            ],
            "then": [{
                    "do": "add",
                    "value": "2"
                }
            ]
        }, {
            "name": "Increase price by 20% if weight is between 6 and 11",
            "predicates": [{
                    "field": "weight",
                    "operator": ">=",
                    "value": "6"
                }, {
                    "field": "weight",
                    "operator": "<",
                    "value": "11"
                }
            ],
            "then": [{
                    "do": "add_percentage",
                    "value": "20"
                }
            ]
        }
    ]
}

I know how to read data from JSON but I have no idea how to generate dynamic if statements using it. I have found eval but I don't know if using it is a correct approach.

I will appreciate any help and tips