Why doesn't Yandex index the title?

Question

Search Engine Optimization

There are two files: header.php and index.php Header
file:
<title><? echo $title;?></title>
Index file:

<?php
    $title = "Мой тайтл";
    include( "{$_SERVER['DOCUMENT_ROOT']}/header.php");
?>

If you open the page code, then the title is there. But Yandex does not index it. What could be the problem ?

Answer 1

https://webmaster.yandex.ru/tools/server-response/
Maybe you have invalid HTML?

Answer 2

The title of the snippet in Yandex is not always taken from the html title of the title page and it is query dependent. What is now displayed in the search results instead of the title?