I\'m trying to set a variable that should be accessible from outside PHP. Ideally this should be a local variable, but environment variables are also welcome.
First,
Environment variables that are exported are only available in child processes.
So you'll be able to set an environment variable and then spawn a child process. The environment variable will be visible in that child process. However setting it in php and then launching a successive process (echo, in your example above) won't work.
If you set the variable and then spawn/exec a new process, it should be visible in that new process.