title: PHP - Replace Letters in a String
body: <p>I am confused by str_replace.
  Here is the thing: <br />
  For example, I have a string:</p>
  
  <pre><code>$string = 'ababcd';
  </code></pre>
  
  <p>Then I want to replace all 'a' to 'b', all 'b' to 'c' and all 'c' to 'a'. My desired result is:</p>
  
  <pre><code>bcbcad
  </code></pre>
  
  <p>But my actual result is:</p>
  
  <pre><code>aaaaad
  </code></pre>
  
  <p>Here is my code:</p>
  
  <pre><code>$string='ababcd';
  $string=str_replace(['a', 'b', 'c'], ['b', 'c', 'a'], $string);
  echo $string;
  </code></pre>
  
  <p>Please help me!!!</p>