How to change the content of the .html page using an external php file?

Question

PHP

Actually, there is such a task:
You need to write a parser in php, which will parse the page on a third-party server once an hour, after which, based on the result of this parsing, you need to replace the code fragment in the .html page.
I will write the parsing and the daemon itself, but the main snag is changing the code in the .html file using an external .php parser.
For simplicity, I just ask you to help me do this thing:
Let's say we have a .html page with the content "a" and the means of the .php file need to change this "a" to "b" ...
I tried to describe it as clearly as possible. Thanks in advance for any possible help.

Answer 1

$html = file_get_contents('my-page.html');
$result = str_replace('а', 'б', $html);

Answer 2

why such difficulties? daemon, php...
bash+curl/wget+sed+cron