No, this is not an April Fool's Day gag...but probably a very sly in-joke for Doctor Who fans.
In "Insecurity," an episode of the Cartoon Network animated series Young Justice, a very familiar-looking blue police box made a brief appearance in Star City, home to DC Comics character Green Arrow. The scene featured Green Arrow and his new sidekick Artemis approaching a police box in a dark alley, something very out of place in America let alone the 21st century.
We quickly discover that the police box is actually a disguised transporter device to allow Artemis to travel to Young Justice's headquarters located in Mount Justice. The police box isn't bigger on the inside like the TARDIS, but a bright light shines through the windows upon activation.
"Insecurity" was written by former Young Justice comic book writer Peter David, a longtime Doctor Who fan who also wrote the short story "One Fateful Knight" for the Doctor Who book anthology Short Trips: The Quality of Leadership.
If you'd like to view the scene, you can watch it below thanks to the kindness of YouTube user KelvingtonDemoReels...