title: Replace character with special characters
body: <p>I have something like this :</p>
  
  <pre><code>$x = "#frac {2} {3}";
  </code></pre>
  
  <p>I try this code but is doesnt work:</p>
  
  <pre><code>$x = str_replace("#","\"",$x);
  </code></pre>
  
  <p>I want to replace # with with \ in this string. But I cant use str_replace.</p>
  
  <p>any help?</p>