GET params is a plugin providing your pages and posts with shortcodes allowing you to display GET parameters from the current URL in pages and posts, or show/hide content depending on GET param values.
As an example, I use this plugin so that I can show personalized installation instructions to users of another piece of software I wrote, simply by directing them to a URL on my WordPress site, along with GET variables chosen to ensure that custom fields and sections display exactly as required for them to install my software.
See FAQ if you are having problems under WordPress 4.2.3
Examples follow, assuming the user goes to your post page http://example.com/post/3/?paramname=showme
Displaying GET parameters directly
[display-get-param name="paramname"]
Shows the value of GET named paramname (‘showme’ in the example URL), or “blank value” if none given.
[display-get-param name="paramname" default="Paramname was blank"]
Shows the value of GET named paramname, or “Paramname was blank” if none
Controlling display of enclosed content depending on GET parameter values
[display-if-get name="myparam"]
This enclosed content only shows if myparam is passed as a GET param (with any value)
[/display-if-get]
Another example specifying a value to match:
[display-if-get name="myparam" value="true"]
This content only shows if myparam is passed as a GET param and equals "true"
[/display-if-get]
Inverting the criteria
The plugin also contains an opposite to display-if-get, called display-if-not-get.
display-if-not-get content will display only in all cases where display-if-get with the same parameters would NOT show.
This means that display-if-not-get content will also show if the named parameter does not exist at all in the URL query string.
If/else blocks
This is essentially possible by using a combination of display-if-get and display-if-not-get with the same parameters.
[display-if-get name="opt" value="1"]
You chose option 1 - URL contains /?opt=1.
[/display-if-get]
[display-if-not-get name="opt" value="1"]
You chose an option that is not option 1.
[/display-if-not-get]