Custom PHP Error pages w\same url

[fileserverdirect]

3RD TIME I REWRITE THIS POST [I hate Macs and there stupid controls]
-----
Hi guys!
Is there any way that I can have a custom PHP error page while keeping the same url. I have custom error page already but it changes the url to mywebsite.com/error.htm instead of a php page keeping the same url such as mywebsite.com/678tgihf.yerw7 Is there any server side stuff (I have my own server) or httpd.conf or .htaccess?
Thanks,

[Leafy]

Use Apache's RewriteRule.