Oops, I missed the point about the porch.You probably wouldn't need
something as complex as a laser, but just a blinking "infrared" led, a
mirror, and a pickup. The blinking would be used as a synchronous
signal, and that would effectively eliminate stray background pickup.
Basically, 40-khz stimulation and TV IR-remote detector, although those
are probably much more sensitive than you would need.
IR Led will help eliminate regular light problems. Note - normal IR Leds
and sensors operate in near-IR range, about 880-940 nm, while PIR operates
in far-IR range, about 10000 nm [IIRC], so you'd actually be dealing with
light and not heat.