Cannot modify header information - headers already sent by

Question

Не получается перейти на страницу об ошибке(show_error.php).Специально ввел неправильный пароль чтобы вышла эта страница

Выходит ошибка

Warning: Cannot modify header information - headers already sent by (output started at C:\OpenServer\domains\lessons\Forms\connect\connect.php:3) in C:\OpenServer\domains\lessons\Forms\scripts\app_config.php on line 8

В чем проблема:

connect.php:

<?php
    require "/../scripts/app_config.php";
    mysql_connect(DATABASE_HOST , USERNAME, PASSWORD)
        or handle_error("Ошибка подключения к базе данных " , mysql_error());

    echo "Connected to MySQL!!!";

    mysql_select_db(DATABASE_NAME)
    or die("There was a problem to connect to database with name " . DATABASE_NAME);

    echo "Connected to database " . DATABASE_NAME;

    $tables = "SHOW TABLES;";

    $result = mysql_query($tables);
    if (!$result){
        die("Error to listing the tables with " . $tables);
    }
    else{
        echo "<br/>Tables in database:";
        echo "<ul>";
            while ($row = mysql_fetch_row($result)){
                echo "<li>Table: {$row[0]}</li>";
            }
        echo "</ul>";
    }
?>

app_config.php:

<?php
    define("DEBUG_MODE",true);
    define("DATABASE_HOST", "localhost");
    define("USERNAME", "root");
    define("PASSWORD", "1");
    define("DATABASE_NAME", "lessons");
    function handle_error($user_error_message, $system_error_message){
        header('Location:show_error.php?error_message={$user_error_message}&system_error_message={$system_error_message}');
        exit();
}
    function debug_print($message){
      if (DEBUG_MODE){
          echo $message;
      }
  }
?>