title: PHP strip punctuation
body: <p>Let's say I have this:</p>
  
  <pre><code>$hello = "Hello, is StackOverflow a helpful website!? Yes!";
  </code></pre>
  
  <p>and I want to strip punctuation so it outputs as:</p>
  
  <pre><code>hello_is_stackoverflow_a_helpful_website_yes
  </code></pre>
  
  <p>How can I do that?</p>