[PHP-INSTALL] Help with missing session_write_close ( ) function

Complete PHP newb here.
 
I have installed PHP 4.3.9 on a FreeBSD 5.2.1 Release box.  All appears to be 
working fine.  My PHP pages on this server have no trouble displaying, but my 
httpd-error log is filling up with the following message:
 
[error] PHP Fatal error:  Call to undefined function:  session_write_close()
 
In my research I have found that my version of PHP should have this function, 
so how do I go about making my server aware of this fact and/or install the 
function by itself?
 
All help is greatly appreciated!
 
-Jamin