There is a compiler.assign.php plugin in Smarty 2.6.19. Its contents can be looked here: http://smarty-php.googlecode.com/svn/branches/Smarty2Dev/libs/plugins/compiler.assign.php
Sometimes PHP halts on the line where this file is included. No errors thrown. But if I make any change in the file (i.e. add a character in a commented line), it starts working. If I remove added text, it halts again. Halting can be reproduced only when file contents are original. If I add one character in one place and remove one in another, the script will work, despite its size is the same as original.
Restarting Apache helps. So this is an Apache+PHP problem.
I was adding and removing characters in the comments only, so they couldn't affect PHP. I am confused very much by this weird problem.
The problem occurs under Apache 2.2.24, PHP 5.3.23 and Linux 2.6.18. Didn't check another environments.