I cannot get the information of response from iPhone device in Phase2

I am about to create a structure that make the local server control the iphone just like「BlackBerry Enterprise for BlackBerry device」.I got the 「iPhone OS Enterprise Deployment Guide」by using google search engine. I have finished the first step 「Phase1-Begin Enrollment」,enrollment begins with me using Safari to access the URL of the profile distribution service I have created,the xml file as bellow:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>PayloadContent</key>
<dict>
<key>DeviceAttributes</key>
<array>
<string>UDID</string>
<string>IMEI</string>
<string>ICCID</string>
<string>VERSION</string>
<string>PRODUCT</string>
</array>
<key>URL</key>
<string> http://192.168.2.157/iphone/index.php</string>
<key>challenge</key>
<string>abcdefgh</string>
</dict>
<key>PayloadDescription</key>
<string>Fhase1</string>
<key>PayloadDisplayName</key>
<string>Prof</string>
<key>PayloadIdentifier</key>
<string>jp.valueengine.prof</string>
<key>PayloadType</key>
<string>Profile Service</string>
<key>PayloadUUID</key>
<string>9CF421B3-9853-4454-BC8A-982CBD3CE07C</string>
<key>PayloadVersion</key>
<integer>1</integer>
</dict>
</plist>

In the「 http://192.168.2.157/iphone/index.php」, 「192.168.2.157」is the web server(Apache), I created「index.php」to receive the HTTP POST info from the iPhone device.

In Phase2, when the iPhone sends the Appleissued certificate back to the web server(Apache) using HTTP Post,I want to get the detail information in the HTTP POST and save it to local, so I wrote the 「index.php」 like follow:
<?php
$str = implode(",", $_POST);
$log = "./limitlog.txt";
if (!$fp = fopen($log,"a")){ exit();}
fputs($fp,$str);
fclose($fp);
?>
The 「limitlog.txt」is saved, but the content is empty. Do I miss something?

Thanks,
ZY