I'm wondering if there is some header weirdness going on.
Can you:
1. Access the page via chrome. Use web inscpector>Network. If you click on the join.php page you should see the full request, status, headers, etc.
2. package Webworks app using -d and deploy to device. Connect to web inspector from Chrome and do the same thing. Do you see anything different?
3. Try the same thing as #2 on Playbook
Somewhere in there, something should stand out as being different. my guess is that your php server is serving up the source vs response for some reason, and that reason has to be part of the request being made to it.
If you can't find it, just post the "request" portion of the results here and i'll take a look as well.
Here is a sample that i grabbed from this page, to give you an idea of what you are looking for:
Request URL:http://supportforums.blackberry.com/t5/forums/replypage/board-id/browser_dev/message-id/29176
Request Method:GET
Status Code:200 OK Request Headersview source
Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Connection:keep-alive
Cookie:__qca=P0-1246504013-1339530061419; __utma=196668671.272471378.1339756336.1339756336.1339756336.1; __utmz=196668671.1339756336.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __atuvc=1%7C34; Survey_3079582=0; LiSESSIONID=206277B8E9F9807DC27C1C0733AF51AF; VISITORID=201675189; lithiumLogin:rim=~28XibHZXltCoAuWfg~6pCrJe3yo5qY8nw-AwCeUTRuJwnNOYptUw9b-R1sp21lcyYpfPTpL5Ec64rKNsqg; LithiumUserInfo=148557; s_sv_sid=502777027332; s_cc=true; VISITOR_BEACON=~2hJ0XSZWlg1cXWGTH~ORJzTw_oQmQMp8Yq9KB3gfY9KvQFzggDFeA-g6eeN00zoQka_3owJVmeQQY6xy80RcrIqjDYhLTEgiaCWNdHzA..; dfa_cookie=rimsupport%2Crimglobal; s_vi=[CS]v1|27EBA99B851D2BBA-6000014AA0132644[CE]; s_sq=%5B%5BB%5D%5D Host:supportforums.blackberry.com
Referer:http://supportforums.blackberry.com/t5/Web-and-WebWorks-Development/Dev-Alpha-deployed-app-AJAX-resp...
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.89 Safari/537.1
