Version Details: Apache 2.0.53, PHP 4.3.10, Mysql 4.1.10, Suse 9.3
Computer: AMD Athlon 2400+, 1gb ram (tested fine)
Domains affected: All that I am hosting
Firewall: Off for debug purposes (everything is logged and no sensitive information on the site, so dont bother trying to hack me please)
Vhost: Name Based & functioning perfectly
Setup: Manual, not using control panel
Connection: Static, Business Connection package, authenticates using PPPoE

Really could use any tips or suggestions here, have ran through countless possibilities.
Basically, DNS resloves fine to the address it should. You can browse to page by IP address or by using the relative domain name of the site fine. However, certain (not all php) scripts from my webserver wont be loaded by a *huge* portion of the internet, but they will be parsed by a small percentage.

At the moment these people can parse all of my pages:
AOL Dialup
Some other level 3 networks were tested at my isp and resolved fine.
Any client of my isp (ACSOL, no relation to AOL)

What it looks like on their end:
Pages load with no errors, php parses even on complicated pages (some of my own, some distros like phpBB, all load fine)

What it looks like on server end:
(access log) IP address, requested document, code 200 and size reported (no errors ~code 200)
(error log) No errors

These people can parse only bland (print/test) php pages, but not anything else (complicated creations of my own and distros such as phpbb)
Cox DSL
Bresnan DSL
Comcast Cable
Quest DSL
(Browsers tested: Firefox, IE, Portable Firefox, Safari, on operating systems Windows XP and MAC)

What its look like on their end:
Connect to host
Wait for Response
Timout and leave no errors (even with E_ALL) - just a blank page loaded

What it looks like on server end:
(access log) IP address, requested document, code 200 and size reported (no errors ~code 200)
(error log) No errors

Ive tried reinstalling php, different router configurations (right now I do not have a LAN address for diagnostic purposes, im plugged directly into an internet router with WAN ips from my bank assigned to respective NIC's) have also tried isolating css as an issue (doesnt appear to be) routers as an issue, apache as an issue, im just completely stumped.

Thanks

Have you tried timing execution of pages using http://us2.php.net/function.microtime ? Does ANYTHING get sent back to the client, or is there no data returned at all?

No, i have not tried timing; however absolutly nothing gets sent to the client.
No errors on either side reported, just a blank page load (server reports code 200 and size, client never shows anything)

no data/errors are returned using every kind of php error reporting either.

ie.. Connecting to host (insert domain)
Host Contacted
Waiting for response from (insert domain)

And it times out eventually loading a blank page, but again some internet clients dont have any problem at all with complicated php scripts off of my server, and nobody has a problem with very simple php scripts - which has me completely stumped.

That is really odd, sad to say.

Just a helpful update, got the problem fixed by isolating every possible problem, and finally falling back on a special IP routing issue, due to the fact it was the only real change from my old web server (which had no problem)

Makes little sense, but it did fix the problem.

The issue & fix: ISP Changed my connecting WAN IP to dynamic, in the 63 range(when I built a new webserver and wanted to utilize my other addresses). IP Package is static, in the 64. range - so technically my LAN addresses were in the 64 and my WAN ip was in the 63, however the LAN addresses are accessable online.. etc whatever. everything resolved fine even though it probably violated the routing table (these arent expensive ciscos here, which probably is the ultimate problem)
Changing the Connecting IP back to a Static Address in the 64 range fixed the problem

Again, I have little explaination, but I only changed one variable at a time - definitly the IP and only the IP change was the solution.