Modal in PHP with php include

I'm trying to use modal in PHP with the include function but there is an error with the header.php file I guess.

Here is the code for the modal:

<!DOCTYPE html>
<html>
<head>
    <title></title>
    <?php include 'header.php'; ?>
</head>
<body>
<button id="myBtn">Open Modal</button>
<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close">x</span>
    <p>Some text in the Modal..</p>
  </div>

</div>
</body>
</html>

This is what is inside the header.php file:

<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">

<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>

<!-- Latest compiled JavaScript -->
<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" type="text/css" href="/css/style.css" media="screen" />
<script type="text/javascript" src="file.js"></script>