title: Replace first two characters of a string in php
body: <p>I am dealing with a problem of time conversion from <code>12 hr</code> format to <code>24 hour</code> format. </p>
  
  <p>Is there any single function in php to replace the first two characters of a string?</p>
  
  <p><code>str_replace</code> can be used only when I know the substring content to be replaced.</p>