The HTTP/1.1 303 REDIRECT header is redundant; the Web server should generate that automatically.
The problem is here, and there are several:
header("Location:" .$_GET("URL");
To reference an array member, use square brackets, not parentheses.
PHP variable names are case-sensitive. URL and url are not the same thing.
There must be a space between the key and the value in an HTTP header.
You need to read the section of the manual that explains how PHP parses variables inside double-quoted strings.
This should work:
<?
header("Location: $_GET[url]");
?>