[mistwang]

How long does it take to complete?
There are various timeout parameters you need to adjust
"Initial request timeout" for lsphp5
http://www.litespeedtech.com/docs/we...s/#initTimeout

If it takes more than 5 minutes to process, you may need to add environment variable
"LSAPI_MAX_PROCESS_TIME=xxx"
http://www.litespeedtech.com/php-litespeed-sapi.html

If FFMPEG uses a lot of memory, increase "Memory soft/hard limit"
http://www.litespeedtech.com/docs/we.../#memSoftLimit

You should check the /usr/local/apache/conf/error_log and stderr.log for related error messages.

It is better to call the php script directly from cron job instead of going through the web server.