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

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=752627