You can't append the path of the site which threw the error to the URL of the displayed error page using the ErrorDocument directive.
However as you're working with PHP you could use the global $_SERVER["REDIRECT_URL"]
value from within your error handling script.
See http://httpd.apache.org/docs/current/custom-error.html#variables for more information:
Redirecting to another URL can be useful, but only if some information
can be passed which can then be used to explain or log the error
condition more clearly.
To achieve this, when the error redirect is sent, additional
environment variables will be set, which will be generated from the
headers provided to the original request by prepending 'REDIRECT_'
onto the original header name. This provides the error document the
context of the original request.
For example, you might receive, in addition to more usual environment
variables, the following.
REDIRECT_HTTP_ACCEPT=*/*, image/gif, image/jpeg, image/png
REDIRECT_HTTP_USER_AGENT=Mozilla/5.0 Fedora/3.5.8-1.fc12 Firefox/3.5.8
REDIRECT_PATH=.:/bin:/usr/local/bin:/sbin
REDIRECT_QUERY_STRING=
REDIRECT_REMOTE_ADDR=121.345.78.123
REDIRECT_REMOTE_HOST=client.example.com
REDIRECT_SERVER_NAME=www.example.edu
REDIRECT_SERVER_PORT=80
REDIRECT_SERVER_SOFTWARE=Apache/2.2.15
REDIRECT_URL=/cgi-bin/buggy.pl
REDIRECT_ environment variables are created from the environment
variables which existed prior to the redirect. They are renamed with a
REDIRECT_ prefix, i.e., HTTP_USER_AGENT becomes
REDIRECT_HTTP_USER_AGENT.
REDIRECT_URL, REDIRECT_STATUS, and REDIRECT_QUERY_STRING are
guaranteed to be set, and the other headers will be set only if they
existed prior to the error condition.
None of these will be set if the ErrorDocument target is an external
redirect (anything starting with a scheme name like http:, even if it
refers to the same host as the server).