更改PHP网站的标题

I am trying to make a website in PHP (I don't really have any experience in this). What I did was use an existing website that somebody created for me and just went about changing things in the .php files.

I had this in header.php:

<base href="http://old-domain.com/"; />
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
<link rel="stylesheet" type="text/css" href="css/default.css?v=4" />
<link rel="shortcut icon" href="favicon.ico" />

I just changed the 'old-domain' to 'new-domain'.

Now, my homepage of the 'new-domain' looks fine. But all other pages, like new-domain.com/help or new-domain.com/about give me a 404.

On the other hand, when I change the 'new-domain' back to 'old-domain', it displays my old website properly, so I guess this means that the code is alright.

What am I doing wrong? Do I need to set up new-domain.com/help, etc somewhere else?

Thank you.

Just check whether any other file contains the link www.old-domain.com and change it to www.new-domain.com.
I assume the project structure is the same.

because there is no seperate php file for those pages create seperate file for each like help.php and about.php and then include those in header