How to detect URL extensions (like ?this=value) in PHP

Hello, 

right now, I am making a PHP page and I want to know how to make PHP detect what I call 'URL or domain extensions'. 

For example,  if the URL is index.php?code=123 , 

• The URL is 'index.php' and 
• The extension is ?code=123

I want to get the '123' value from the 'code' prefix to a variable so I can use it for other things like

#import 123 to variable 'code'
echo "$code";

Please tell me how this happens. 

Thx. 

P.S. If my English is faulty please forgive me.