i found an iphone in the street what should i do

There is a "Return a lost, locked iPhone - Apple Support" article but it only helps with phones that show a message that says that the iPhone has been reported lost or stolen. Without that message, law enforcement will only be able to act if they (1) accept found property (not all agencies do) and (2) the owner has reported the loss or theft to them including a serial number (the only useful info that a locked phone will reveal). 

You would think that Apple would be able to look up the serial number and return the phone to its owner. But today I was told at the Apple Store in San Mateo, CA that Apple keeps serial number records for only 90 days. So if you find a phone that was sold more than 90 days ago and it's powering on, they can't help. And they won't accept the phone for recycling because the Activation Lock when it's on shows the phone is not yours.

Turn it in to your local police department.