title: Cannot work out a php str_replace() to remove comma
body: <p>In a PHP project, I have:</p>
  
  <pre><code>$string = "1,555";
  
  str_replace(',', '', $string);
  echo $string; //is still 1,555
  </code></pre>
  
  <p>str_replace does not remove the comma. If I var_dump. I get <code>string(5) "1,555"</code></p>
  
  <p>Any ideas? I just simply need to remove the commas so I can compute.</p>