title: How to replace string in form A_B to string in form B_A?
body: <p>I have a string in form:</p>
  
  <pre><code>hello_world
  </code></pre>
  
  <p>Need to have a string in form:</p>
  
  <pre><code>world_hello
  </code></pre>
  
  <p>I know that I probably need to use <code>str_replace</code> function, but how to use it exactly? Or maybe there is a better method?</p>