php - Obtaining the current working directory of a symbolic link

I have an Apache server which serves one site, say example.com, at/var/www/html/, and another site, say othersite.com, at/home/michael/public_html/. I have a symbolic link in the latter directory, which points to a file in the former. Here's an example directory structure, to give some context for what I'm looking to do.

/var/www/html/:
    header.php
    a.php
    mydir/:
        b.php
    

/home/michael/public_html/:
    header.php -> /var/www/html/header.php
    a.php
    mydir/:
        b.php
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header.php includes the linerequire('a.php');. What I'm expecting is, if I'm visiting example.com, thisrequire will load in the file at/var/www/html/a.php, but if I'm on othersite.com, it instead will load/home/michael/public_html/a.php.

For extra clarity, let's say those twoa.php files have the following contents:

/var/www/html/a.php:
echo("I'm on example.com");

/home/michael/public_html/a.php:
echo("I'm on othersite.com");
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If I point my browser to example.com/header.php, I see "I'm on example.com". Similarly, going to othersite.com/header.php, I see "I'm on othersite.com". So far so good.

Here's my problem. The filemydir/b.php, in both sites, has the linerequire('../header.php'), so thatheader.php gets loaded in. Once again, I'm expecting to see the different strings depending on which site I'm visiting. But in my browser, I see "I'm on example.com" when I visit both example.com/mydir/b.php and othersite.com/mydir/b.php.

From reading the documentation for require/include, I'm pretty sure I understand what's happening.

• When I go to othersite.com/header.php, the current working directory is/home/michael/public_html/. So therequire('a.php'); line correctly loads/home/michael/public_html/a.php, because it was able to find it in the current working directory.
• When I go to othersite.com/mydir/b.php, the current working directory is/home/michael/public_html/mydir/. In this casea.php is not in the current working directory, so it doesn't get loaded from there. Instead, PHP checks the calling script's directory fora.php. Since/home/michael/public_html/header.php is a symbolic link to/var/www/html/header.php, it always considers the "calling script's directory" to be/var/www/html/, and so it always loads/var/www/html/a.php.

That was a lot of detail, but I hope everything is clear.

What I want is some way for/home/michael/public_html/a.php to be loaded by othersite.com/mydir/b.php. In my head I think I'm looking for some way of obtaining the parent directory ofheader.php, likedirname(__FILE__), but which can also detect when the script was called by a symbolic link, and return the parent directory of the link. Then, I can justchdir(...) to the directory that I want before callinga.php, and switch back afterwards.

Of course, I'm very open to other suggestions, but would definitely prefer if they only involved changingheader.php, and notmydir/b.php. This is becausemydir/b.php really represents hundreds of pages at several different directory levels, many of which I do not manage, and it wouldn't be feasible to edit all of them.

Is this possible?

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