We know that Milo Murphy's Law takes place in the same universe as Phineas and Ferb. This eveident from the many references to Phineas and Ferb in the later Milo Murphy's Law. But is the reverse true as well? Does Phineas and Ferb ever reference its sequel?
The producers have confirmed that the shows take place in the same universe, and the crossover episode (allegedly premiering in August along with the new season) will cement that into continuity.
The P&F Wiki has a large (and growing) list of references to the show on Milo. Some examples:
Dakota and Cavendish appear in Phineas and Ferb costumes in "The Last Halloween" song. There's a kid dressed as Perry in the same episode.
In "Rooting for the Enemy", a background character says, "It's like Football X-7," referencing the football game played in "My Fair Goalie".
In "Murphy's Lard", it is revealed that P&F's Rollercoaster (From the episode of the same name) ended up in Melissa's backyard, traumatizing her for life.
In "Worked Day", Cavendish has a map of Danville when he and Dakota try and make sure a shipment truck full of pistachios make it to the pistachio warehouse.