php - Why am I getting these undefined index errors?

Why am I having undefined index errors here on my code when I add the sort feature? These are the errors I'm getting

Notice: Undefined index: sort in C:\xampp\htdocs\how are things\admin panel\view.php on line 58

Notice: Undefined index: sort in C:\xampp\htdocs\how are things\admin panel\view.php on line 61

Notice: Undefined index: sort in C:\xampp\htdocs\how are things\admin panel\view.php on line 64

Notice: Undefined index: sort in C:\xampp\htdocs\how are things\admin panel\view.php on line 67

Notice: Undefined index: sort in C:\xampp\htdocs\how are things\admin panel\view.php on line 70

Notice: Undefined index: sort in C:\xampp\htdocs\how are things\admin panel\view.php on line 73

Notice: Undefined index: sort in C:\xampp\htdocs\how are things\admin panel\view.php on line 76

This is the code on the lines 58 61 64 67 70 73 76

$result = "SELECT * FROM customers";

    if($_GET['sort'] == 'first_name'){
     $result .= " ORDER BY first_name";
    }
   else if($_GET['sort'] == 'last_name'){
     $result .= " ORDER BY last_name";
    }
     else if($_GET['sort'] == 'address'){
     $result .= " ORDER BY address";
    }
     else if($_GET['sort'] == 'phone_number'){

     $result .= " ORDER BY phone_number";
    }
     else if($_GET['sort'] == 'email'){
     $result .= " ORDER BY email";
    }
     else if($_GET['sort'] == 'city'){
     $result .= " ORDER BY city";
    }
     else if($_GET['sort'] == 'country'){
     $result .= " ORDER BY country";

    }
$result = mysql_query($result) or die(mysql_error());

Answer

you should check if sort is set and then use it. maybe something like this would work for you:

    $allowedSorts = array('first_name', 'last_name', 'address','phone_number', 'email', 'city', 'country');

    $sort = isset($_GET['sort']) ? $_GET['sort'] : '';
    $result = "SELECT * FROM customers";

    if(in_array($sort, $allowedSorts)){
         $result .= " ORDER BY {$sort}";
    }

    $result = mysql_query($result) or die(mysql_error());