Control Arduino with Web Browser

Someone created some arduino code for me that works. It basically reads co2 levels. And allows you to set a threshold for the co2 level. So if co2 is above a certain amount then it turns of the solenoid valve. When it's below the set value then it opens the valve to allow more co2 in. Pretty simple. Just need a php script so someone could read the co2 from their web browser and also set the co2 threshold. Let's say the threshold is 1%. They could change it to 2%. I want something simple that works. Once it works then we will add on features like setting how long you want a certain threshold. For example 10 minutes at 1% then 10 minutes at 2% etc....but that will be the next project if this one is successful. If you have any other ideas just let me know. Thanks!