own CONSTANT?

[eluser]Josssi[/eluser]
Thank you very much for reaction ,
this reasont are just for one cycle so i have to use an other reason. You now, my system it is like in shop. If owner has good day, so as an admin can set -10% for every customer. This value 10 i wanted to store in config file without creating a database. I thought that it is possible to change value of CONSTANT. I thought that i can to set static value. Because if owner will be in the next day angry, so bonus -10% will be deleted, an I need in this contant have again value 0