Declaring global variable inside a function

declare inside a function doesn’t work as expected.
I needed read-only global variables declared in a function.
I tried this inside a function but it didn’t work:

declare -r MY_VAR=1

But this didn’t work. Using the readonly command did:

func() {
    readonly MY_VAR=1
}
func
echo $MY_VAR
MY_VAR=2

This will print 1 and give the error “MY_VAR: readonly variable” for the second assignment.