The trigger is executed on the MySQL server, not on the PHP one (even if those are both on the same machine).
So, I would say this is not quite possible -- at least not simply.
Still, considering this entry from the MySQL FAQ on Triggers :
23.5.11: Can triggers call an external application through a UDF?
Yes. For example, a trigger could invoke the
sys_exec()UDF available here: https://github.com/mysqludf/lib_mysqludf_sys#readme
So, there might be a way via an UDF function that would launch the php executable/script. Not that easy, but seems possible. ;-)