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.

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.

  • 1
    Is this really how your script is laid out? I mean, you echo the variable right after the variable declaration and you get this result? I am asking because your example, as posted is impossible, at least with the php I am used to. Feb 16, 2012 at 4:45
  • 1
    You might want to check your php.ini file to look for any sort of settings that might have to do with domains or URL settings? Haven't seen something like this, before. Like @stefgosselin said, is this your whole script? Nothing else is being included, right?
    – summea
    Feb 16, 2012 at 6:15
  • Yes, this is also the entire script. No files included. I am not familiar with anything that could do this. But it is. Feb 16, 2012 at 19:13

1 Answer 1


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

Example filter handler code:

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) {

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

    return Apache2::Const::OK;

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

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