Joomla! Documentation™

As reported by auciker Because of a change made to improve security, login with is redirection will does not work for PHP 4 sites. This may also impact other areas where there is redirection, specifically polls and mailto.

To fix this:

• Add this to the end of /libraries/joomla/utilities/compat/php50x.php

 if(!function_exists('stripos')) {
 function stripos($haystack, $needle, $offset = 0) {
  return strpos(strtolower($haystack), strtolower($needle), $offset);
 }
}

• OR - Upgrade to PHP 5. PHP 4 end of life was August 8, 2008. There will not be any more PHP 4 releases, even for security. (Switching PHP versions may cause problems with some 3rd Party Plugins - be sure to test thoroughly).