When I run php file,browser shows blank and the error.log file shows following error
[Tue Mar 11 18:51:37 2014] [error] [client 127.0.0.1] PHP Fatal error: Uncaught --> Smarty: unable to write file templates_c/wrt531f0de1310f36.31049288 <-- \n thrown in /var/www/forum/modules/smarty/sysplugins/smarty_internal_write_file.php on line 44
That probably means that either the templates_c directory doesn't exist, or the script has unsufficient rights to store files in that folder.
You can make that folder writeable by making adjustments to the permissions using chmod. You can do this in most of today's FTP clients. The directory needs to be writable by the user that runs the PHP code (usually www, apache, nobody).
If unsure, set the directory's permissions to 0777 and that should do the trick. But keep in mind that when you set it to 0777, it is world-writeable, so depending on your setup, you might want to check for the right value.