I'm curious to know if there is a way to have URLs be intercepted by a PHP page or some other server side logic such that if the URL "www.mysite.com/about/mission_statement" is queried but I don't have a page at "about/mission_statement", that I can serve up the content from, say, "some/other/folder/with/pages".
Does anybody know if this is possible, or do I have to ensure that I have a page available for every url address I wish to handle?
You cannot do that in PHP directly because a PHP page is invoked in order to process a given URL, but you can do that in your web server.
Assuming Apache have a look at URL rewriting
http://httpd.apache.org/docs/2.2/rewrite/
In particular for your case, the subtopic
http://httpd.apache.org/docs/2.2/rewrite/remapping.html
is relevant.
IIS has a similar capability.
Again presuming your using Apache, then you have a number of options:
Use a Rewrite rule (as specified in another answer) for specific pages/content or include a line in your .htaccess file that redirects any requests that don't exist (404's) to a specific location/file of your choice.
This is usually envoked using the following code in an .htaccess file either in your docroot directory or other directory of choice:
ErrorDocument 404 /whatever/path/to/file.php
If your using dynamic pages in PHP keyed by, say ID or some other dynamic value, EG:
http://www.somedomain.com/page.php?id=4
then you need to create a function that redirects to your page of choice if the content doesn't match or exist.