PHP Warning:  Undefined array key "header" in /home//themes/views/header.php on line 2

<?php 
    if ($_GET["header"]) {
        if ($_GET["header"]=="1") {
            include('header-1.php');
        }
        if ($_GET["header"]=="2") {
            include('header-2.php');
        }
        if ($_GET["header"]=="3") {
            include('header-3.php');
        }
        if ($_GET["header"]=="4") {
            include('header-4.php');
        }
        if ($_GET["header"]=="5") {
            include('header-5.php');
        }
        if ($_GET["header"]=="6") {
            include('header-6.php');
        }
        if ($_GET["header"]=="7") {
            include('header-7.php');
        }
    } else {
        if ($site_theme["header"]=="header-1") {
            include('header-1.php');
        }
        if ($site_theme["header"]=="header-2") {
            include('header-2.php');
        }
        if ($site_theme["header"]=="header-3") {
            include('header-3.php');
        }
        if ($site_theme["header"]=="header-4") {
            include('header-4.php');
        }
        if ($site_theme["header"]=="header-5") {
            include('header-5.php');
        }
        if ($site_theme["header"]=="header-6") {
            include('header-6.php');
        }
        if ($site_theme["header"]=="header-7") {
            include('header-7.php');
        }  
    }
?>