How to get main actitvity variable value in resource strings.xml

You can't add to the strings.xml file at runtime. What exactly are you trying to do that would necessitate such? Surely we can recommend a better solution.

You actually can write to a file so you could get strings.xml and write it to the phone's storage (you'll have to ask the user for storage permission). Then you can read in the xml, make changes and write it out.
You mentioned PHP. Is this PHP file running on a server someplace? You want to retrieve the value by calling the PHP file and then write it out to the xml? Perhaps local storage in a database might be more appropriate (secure). I would not suggest that you do this with your resources file. If you want to store a value that you read at runtime then create your own file or use a database.