From Observium

Make sure these options are set :

options {
	chain_hostnames(0);
	keep_hostname(1);
	use_dns(no);
};

Use this as destination in syslog-ng.conf, change syslog.php path to match yours

source s_net {
	udp();
};

destination d_observium { 
	program("/opt/observium/syslog.php" template ("$HOST||$FACILITY||$PRIORITY||$LEVEL||$TAG||$YEAR-$MONTH-$DAY $HOUR:$MIN:$SEC||$MSG||$PROGRAM\n") template-escape(yes));  
};
log {
	source(s_net);
	destination(d_observium);
};