Description:
------------
Substr returns "" on string_length + 1 instead of "false"

Test script:
---------------
<?php

var_dump(substr("test", 3));  // "t" in PHP 5,  "t" in PHP 7.*
var_dump(substr("test1", 4));  // "1" in PHP 5,  "1" in PHP 7.*

// BUG START!!
var_dump(substr("test", 4));  // false in PHP 5,  "" in PHP 7.*
var_dump(substr("test1", 5));  // false in PHP 5,  "" in PHP 7.*
// BUG END

var_dump(substr("test", 5));  // false in PHP 5,  false in PHP 7.* 
var_dump(substr("test1", 6));  // false in PHP 5,  false in PHP 7.*


Expected result:
----------------
var_dump(substr("test", 4));  // false
var_dump(substr("test1", 5));  // false

Actual result:
--------------
var_dump(substr("test", 4));  // "" in PHP 7.*
var_dump(substr("test1", 5));  // "" in PHP 7.*