I have installed xampp to Ubuntu 12.04. I have put my project in the folder /opt/lampp/htdocs/project_is_here
When I type in the browser localhost/soap/php
One possible reason is that you are using Virtual host.
In that case, use this command in your terminal
sudo nano /opt/lampp/etc/extra/httpd-vhosts.conf
Then add this block of code at the end of the file
DocumentRoot "/opt/lampp/htdocs"
ServerName localhost
ServerAlias localhost
ErrorLog "logs/localhost-error_log"
CustomLog "logs/localhost-access_log" common
Finally restart XAMPP
sudo /opt/lampp/lampp restart