Create Custom Global variable

Hello,

I need to create a new global variable that can be changed in the General setting menu. What I am looking for is a new input in the form that allows me to change a variable that I can access in my php in index.php.

The variable is called called current issue and holds a string that represents an existing tag. I would like to add this variable into my query call in the loop so I can control which posts are shown depending on the tag.

Im not sure if this makes complete sense but if you have any ideas please let me know. I got stuck with the add_settings_field() function as I couldn’t figure out which file to place it in.

thanks

ben