/etc/php5/apache2/php.ini
log_errors = On
error_log = syslog

/etc/syslog-ng/syslog-ng.conf
destination php { file("/var/log/php.log" owner(root) group(root) perm(0620)); };
log { source(s_src); filter(f_dbg); destination(php); };

Logs will appear in /var/log/php.log