Some variations include wild onion instead of cumin, while others include radhuni seed. However, panch phoron is a slight misnomer when applied to blends that include radhuni (Trachyspermum roxburghianum syn. Carum roxburghianum), since in Oriya or Bengali panch phoran literally means "five spices".
Wild onion seed is a misnomer applied to nigella seeds, as the seeds from the two plants resemble one another. They do not replace anything. The only variation (though maybe only the true panch phoran in Bengal) is the substitution of radhuni for mustard seed, everything else remains the same.
In the tradition of Oriya and Bengali cuisine, one usually first fries the panch phoron in cooking oil or ghee, which causes them to start popping immediately. At this point, one adds vegetables (especially potatoes), lentils, or fish to the cooking vessel to coat with the spice mixture.
In Bengal, where this mixture was first created, a spice called radhuni is used, not mustard seed. Celery seed would be a good replacement for radhuni outside of Bengal, as radhuni is rarely available outside of Bengal.
Panch phoron is usually only used to season fish.