Description:
------------
The function stripos() returns FALSE when a empty string is used.
Consider the example below, an empty string is used and it is found at position 0. It is misleading (at least theoretically) that an empty string is not found inside another string.

Reproduce code:
---------------
<?php
var_dump(stripos('test', 'te'));
var_dump(stripos('test', 't'));
var_dump(stripos('test', ''));
var_dump(stripos('test', 'es', 1));
var_dump(stripos('test', 'e', 1));
var_dump(stripos('test', '', 1));
var_dump(stripos('test', '', 2));
var_dump(stripos('', ''));
?>


Expected result:
----------------
int(0)
int(0)
int(0)
int(1)
int(1)
int(1)
int(2)
int(0)

Actual result:
--------------
int(0)
int(0)
bool(false)
int(1)
int(1)
bool(false)
bool(false)
bool(false)