Description:
------------
The extract() function with EXTR_PREFIX_SAME option specified prefixes variable name which is an empty string, although such variable doesn't exists in the scope where extract is called.

Manual says:
"EXTR_PREFIX_SAME
If there is a collision, prefix the variable name with prefix."

And this is not true for an empty string.
A bug is possibly here (!!!):
-- array.c ----------------------------
case EXTR_PREFIX_SAME:
if (!var_exists)
smart_str_appendl(&final_name, var_name, var_name_len);
/* break omitted intentionally */

case EXTR_PREFIX_ALL:
if (final_name.len == 0 !!!) { ...
break;
---------------------------------------

Reproduce code:
---------------
function f()
{
  extract(array("" => 1),EXTR_PREFIX_SAME,"prefix");
  print_r(get_defined_vars());
{
 
f();


Expected result:
----------------
Array
(
)



Actual result:
--------------
Array
(
    [prefix_] => 1
)