Is Drupal 6 compatible with php5.3?
System requirements says:


  Although PHP 5.3 is supported by Drupal 6.14 core and higher (see the release notes for 6.14), some contributed modules may not be compatible with PHP 5.3, and some PHP 5.3 configurations still show warnings with Drupal 6.14; work is ongoing to resolve the latter in #360605: PHP 5.3 Compatibility.


The issue linked in the requirements page is actually marked as fixed; this means that Drupal core is completely compatible with PHP 5.3 since Drupal 6.14.