title: delete string first character with index strange behavior
body: <p>If I do: </p>
  
  <pre><code>$str = "+12";
  $str[0] = "-"; // -12
  </code></pre>
  
  <p>But when I want to remove character like:</p>
  
  <pre><code>$str[0] = '';
  </code></pre>
  
  <p>Dumping it outputs black clubs question mark: </p>
  
  <pre><code>�12
  </code></pre>
  
  <p>How this works?</p>