Including an php which is including another php - include

Question

I am new to php language but I need the include() function to separate my site into different PHPs in order to call it again on different pages.All worked fine until I came to include a PHP which includes another PHP . Here's the code in html file :
<?php
set_include_path('contents');
include 'index_page.php';
?>
index_page.php which includes sidebar.php :
<?php
echo
'<div class="body2">
<div class="content">
<h1>text</h1>
<img src="images/homeimage.jpg">
<p>some text</div> '. include "sidebar.php"; '</div>';
?>
sidebar.php:
<?php
echo
'<div id="sidebar">
<ul>
<li><div class="arrow-up"></div>اشترك معنا</li>
<li>text</li>
<li>text</li>
<li>text</li>
<li>text</li>
<li>text</li>
</ul>
</div>';
?>
The result that appears in the browser was that the sidebar.php was included before the text as if the code was like that :
<?php
echo
include "sidebar.php";.'<div class="body2">
<div class="content">
<h1>text</h1>
<img src="images/homeimage.jpg">
<p>some text</div> </div>';
?>
Any solutions ??