To set up a custom log file via the syslog daemon (FreeBSD in this case)...
Add to /etc/syslog.conf a line that says all errors from the httpd process are to go to a file called (for example) /var/log/httpd-php.log
!httpd
*.* {tab} /var/log/httpd-php.log
Note the tab, being a tab character! Next create a blank file to be written to. I'm sure there are 1e+6 ways to do this, but I choose
# cat > httpd-php.log << EOF
? EOF
Finally find your syslog daemon and send it a sighup to inform it of the change:
# ps ax | grep syslogd
133 ?? Ss 0:07.23 syslogd -s
# kill -1 133
Et voila! Php syslog calls will now arrive in /var/log/httpd-php.log