What would cause PHP variables to be rewritten by

I was given a VM at my company to install web software on. But I came across a rather bizarre issue where PHP variables would be overwritten (rewritten) by the server if they matched a specific pattern. What could rewrite PHP variables like this?

The following is as an entire standalone script.

<?php
$foo = 'b.domain.com';
echo $foo; // 'dev01.sandbox.b.domain.com'

$bar = 'dev01.sandbox.domain.com';
echo $bar; // 'dev01.sandbox.sandbox.domain.com'

$var = 'b.domainfoo.com';
echo $var; // 'b.domainfoo.com' (not overwritten because it didn't match whatever RegEx has been set)
?>

Essentially any variable which contains a subdomain and matches on the domain name would be rewritten. This isn't something mod_rewrite would be able to touch, so it has to be something at the server level that is parsing out PHP and rewriting a string if it matches a RegEx.

Output overwriting is possible within Apache by using mod_perl: PerlOutputFilterHandler.

The following could be added to an apache.conf to set the output filter:

<FilesMatch "\.(html?|php|xml|css)$">
    PerlSetVar Filter On
    PerlHandler MyApache2::FilterDomain
    PerlOutputFilterHandler MyApache2::FilterDomain
</FilesMatch>

Example filter handler code:

#file:MyApache2/FilterDomain.pm
#--------------------------------
package MyApache2::FilterDomain;

use strict;
use warnings;

use Apache2::Filter();
use Apache2::RequestRec();
use APR::Table();

use Apache2::Const -compile => qw(OK);

use constant BUFF_LEN => 1024;

sub handler {
    my $f = shift;
    my @hostname = split(/\./, $f->r->hostname);
    my $new_hostname = $hostname[0].".".$hostname[1];

    unless ($f->ctx) {
        $f->r->headers_out->unset('Content-Length');
        $f->ctx(1);
    }

    while ($f->read(my $buffer, BUFF_LEN)) {
        $buffer =~ s/([a-z0-9]+)+\.domain\./$new_hostname\.$1.domain\./g;   
        $f->print($buffer);
    }

    return Apache2::Const::OK;
}
1;

More on Apache mod_perl filters can be found here: mod_perl: Input and Output Filters