Fix a php warning

[jvoisin]

This should close #8670

Verification

List the steps needed to make sure this thing works

• Start msfconsole
• Use whatever exploit to uses a php payload
• exploit
• Check that no warning appears anymore in the web server logs.

[busterb]

Good catch, thanks @jvoisin

[busterb]

Release Notes

This fix resolves syntax warnings in PHP payloads about uninitialized constants, which can be used as an IOC during exploitation. The warnings are no longer triggered.