Major Zabbix combines code released under the GPL with code released under
the PHP license in
src/libs/zbxsys/mutexs.c
(The PHP license is not included in the header, but somewhere below.)
As the GPL and PHP license are incompatible[1], this makes the software
not distributable.
[1]: https://www.gnu.org/licenses/license-list.html#PHP-3.01
Originally reported in