While troubleshooting some PHP Curl issues, I found and used respondto.it (and later http://requestb.in/) which allows you to create a dummy webservice endpoint which reveals the full request made to it by your code. (Note: both of those services are offline now, see my newer article linked below for a more recent list.)
An even simpler use case would be a webservice that simply returned data about the request directly to the calling application. I just created such a simple echo webservice on my scooterlabs.com domain.
Update 2012-03-23: Added XML response example as well (scooterlabs.com/echo.xml).
Update 2019-04-30: Updated to reflect services which have gone offline. I’ve also written up a quick recent survey of HTTP inspector services which are still available.
Plain Text Example
$ curl http://scooterlabs.com/echo
Array
(
[method] => GET
[headers] => Array
(
[User-Agent] => curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8r zlib/1.2.3
[Host] => scooterlabs.com
[Accept] => */*
)
[request] => Array
(
[foo] => bar
)
[client_ip] => 68.125.160.82
[time_utc] => 2012-01-08T21:33:28+0000
[info] => Echo service from Scooterlabs (http://www.scooterlabs.com)
)
Plain text response with public IP address:
$ curl http://scooterlabs.com/echo?ip
10.11.12.13
JSON Example
$ curl --silent curl http://scooterlabs.com/echo.json?foo=bar | json_xs
{
"info" : "Echo service from Scooterlabs (http://www.scooterlabs.com)",
"request" : {
"foo" : "bar"
},
"headers" : {
"User-Agent" : "curl/7.21.3 (i386-portbld-freebsd7.3) libcurl/7.21.3 OpenSSL/1.0.0e zlib/1.2.3 libidn/1.22",
"Accept" : "*/*",
"Host" : "scooterlabs.com"
},
"client_ip" : "66.39.158.129",
"time_utc" : "2012-01-08T22:07:54+0000",
"method" : "GET"
}
XML Example
$ curl --silent http://scooterlabs.com/echo.xml?foo=bar | xml_pp
<?xml version="1.0"?>
<echo>
<method>GET</method>
<headers>
<User-Agent>curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8r zlib/1.2.3</User-Agent>
<Host>scooterlabs.com</Host>
<Accept>*/*</Accept>
</headers>
<request>
<foo>bar</foo>
</request>
<client_ip>68.122.10.221</client_ip>
<time_utc>2012-03-24T17:05:49+0000</time_utc>
<info>Echo service from Scooterlabs (http://www.scooterlabs.com)</info>
</echo>
Source
Source code is up on Github: https://github.com/bcantoni/echotest. If anyone has any comments or feedback, let me know here or on Github.