If you need to replace strings in the output of your PHP scripts before they are sent to the browser, you can use Apache's string substitution features.
To have Apache perform a string replacement, add the following to the app's .htaccess file:
AddOutputFilterByType SUBSTITUTE text/html Substitute s/sitename.example.com/sitename.com/ni
That would replace every instance of sitename.example.com with sitename.com before any page is sent to the browser. The flags at the end (ni) tell Apache to use a fixed string rather than a regular expression (n) and to do a case-insensitive search (i).
To perform more than one substitution, you only need to add more Substitute directives. You do not need to repeat the line AddOutputFilterByType SUBSTITUTE text/html. For example, two substitutions would look like:
AddOutputFilterByType SUBSTITUTE text/html Substitute s/sitename.example.com/sitename.com/ni Substitute s/othersite.example.com/othersite.com/ni
You can find more information in the Apache documentation for mod_substitute.