Description:
------------
FILTER_FLAG_STRIP_BACKTICK is ignored unless FILTER_FLAG_STRIP_LOW or FILTER_FLAG_STRIP_HIGH is used.

Test script:
---------------
var_dump(filter_var("``a`b`c``", FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_BACKTICK));
var_dump(filter_var("``a`b`c``", FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_LOW | FILTER_FLAG_STRIP_BACKTICK));
var_dump(filter_var("``a`b`c``", FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_HIGH | FILTER_FLAG_STRIP_BACKTICK));

Expected result:
----------------
string(3) "abc"
string(3) "abc"
string(3) "abc"


Actual result:
--------------
string(9) "``a`b`c``"
string(3) "abc"
string(3) "abc"