Anyone storing them in a DB, or using them in internal fields will likely have a surprise on their hands. Unless they store them as opaque strings anyway, which is the saner thing to do in these situations anyway.
At that point you’re just blowing up storage for no reason. Just use an int if you’re that sure.
Setting a string length to coincidentally the length of a int serialized to a string while doing no other validation on it is…. Just special.