If you use Apache you can do the URL routing via mod_rewrite.
Small example:
RewriteEngine On
RewriteRule ^(dir1)/?(path2)? main.php?dir=$1&path=$2
That'll have any request like
http://yoursite.com/dir1/path1
served by
http://yoursite.com/main.php?dir=dir1&path=path2
More examples here.
The other alternative have every request redirect to a single php file
RewriteEngine On
RewriteRule (.*) main.php?request=$1
and then to do it in code, where you can use a similar approach, by having a set of regular expressions that are matched by some code and then redirected via header() or just internally.
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