title: How to exec a command installed with `npm` command in PHP using `exec()`?
body: <p>In PHP running on Ubuntu, I can run <code>exec('npm -v')</code> and the output is good, 
  but I can't run <code>exec('gitbook xxxx')</code>.</p>
  
  <p>gitbook is a npm package I installed by </p>
  
  <pre><code>npm install gitbook -g
  </code></pre>
  
  <p>I can run <code>gitbook xxxx</code> in the Ubuntu terminal, how can I run it from my PHP code?</p>