Here's a basic PTT script, please feel free to modify and expand. I know it has flaws :)
To use the script you'll need PHP on a webserver accessible by the phones. You'll need to add this to the section of your cnf.xml files as well
<device>
...
<vendorConfig>
...
<webAccess>0</webAccess>
<PushToTalkURL>http://webserver/path/ptt.php</PushToTalkURL>
...
</vendorConfig>
...
</device>
<authenticationURL>http://webserver/path/authorize.php</authenticationURL>
Note: the in SEP.cnf.xml has to be set and should return AUTHORIZED for the user/pwd defined in the script below (see: Setup phone authorization.
You can also use push2talk on other Cisco Device that don't have a Push-to-Talk button. You just add the ptt.php script to your services page, to make it accessible to the user.