When coding PHP together with HTML, it always seems confusing and very painful to look at. The structure is not easy to understand.
Can anyone tell about how to code PHP with HTML in good structure?

When coding PHP together with HTML, it always seems confusing and very painful to look at. The structure is not easy to understand.
Can anyone tell about how to code PHP with HTML in good structure?

There are methodologies out there that help with this issue, such as Model-View-Controller (MVC). MVC seperates your data layer, from your logic, from your presentation (UI) layers. A framework such as CodeIgniter would help you in this case. Even if you weren't going to use an existing framework, seperating out your data models from your business logic and then from your UI objects is relatively easy.
Edit: Let's say you have a shopping cart page. You can have a "view" file called cart.php, and then you can send it information from your other PHP files (the "Controllers"), so that instead of this:
<div id = 'special_price'>
<?php $result = mysql_query("SELECT price FROM pricelists");
$row = mysql_fetch_assoc($result);
echo $row["price"];
?>
</div>
you can do this:
<div id = 'special_price'><?= $price ?></div>
In this scenario, all logic and data access are handled before you attempt to display the page.