Solving Drupal 6 Multibyte String Problem in PHP
If an existing Drupal 6 site encounters the Unicode library error "Multibyte string input conversion in PHP is active and must be disabled" after upgrading PHP, then the following steps may help resolve the issue.
1. In the Drupal folder, open the file that handles the Drupal settings.
2. Scroll to the bottom of the file and add the lines marked "After PHP upgrade" as follows.
// BEGIN CODE
ini_set('mbstring.http_output', 'pass'); //After PHP upgrade
// END CODE
3. Save the file and reload the page.