title: How do I check whether an environment variable is set in PHP?
body: <p>In PHP, how do I test whether an environment variable is set? I would like behavior like this:</p>
  
  <pre><code>// Assuming MYVAR isn't defined yet.
  isset(MYVAR); // returns false
  putenv("MYVAR=foobar");
  isset(MYVAR); // returns true
  </code></pre>