Cross Language Variables - Implementation

Certain Variables I use acrross Javascript and PHP, for example JSON_ON.

I need it set to either 1 on both sides or 0 on both sides...this sets encoding and decoding of structured data.

One way I could implment this is to just have PHP open up the .js file and read the variable.

Is this done? Are there better ways to implement variables across languages?

I only want to have to set the variable in one place, in this case the .js File.