re-read the thread from beginning, got idea what might have been wrong.
this file is used for build lsphp from lsadmin consonle:
/usr/local/lsws/admin/html/utility/build_php/buildconf.inc.php
and it'll update from time to time when new release/build comes out, so not a issue regarding "can't go over 5.4.19"
this wiki
http://www.litespeedtech.com/support/wiki/doku.php?id=litespeed_wiki:multiple_phps_in_control_panel_lsws don't mention lsapi version.
just now I downloaded latest php-5.4.20 source, sapi/litespeed still in 5.5 version. to build with latest lsapi 6.3, please go
https://www.litespeedtech.com/open-source/litespeed-sapi/download to get the latest 6.3, extract it, and replace all *.h and *.c in php-5.4.20/sapi/litespeed/ (and keep other files), then start build lsphp-5.4.20, the lsapi will be latest lsapi 6.3
I think(not verify yet) the GUI in admin console to build lsphp has automated above process.
Sorry for the inconvenience caused.