I don't think it makes any sense to pass an empty string directly to substr and make it behave weirdly. Just wrap it inside an if statement to check if the string is empty if you are using userdata.

In either case, calling sub string on an empty string would return nothing, simply casting it to a string makes it empty:
$string = (string) substr('', 0, 2);

Example: I have string $s and want to short it to 5 characters:

$res = substr($s, 0, 5);

Question: is $res string of boolean?

Then check the return value of substr():

if(($string = substr($old_string, 0, 5)) === false) {
        /* error */
}

There is a million ways to over come this problem, simply check the string first before trying to substring it