I need to create a global constant variable and save
one URL as value.
I must be able to use this variable in any page in the mediawiki.
Try creating a template:
https://www.mediawiki.org/wiki/Help:Templates
In your example, you'd create an article called (say) "Template:My beautiful
URL" that contains
http://whatever. Then in any article, you type:
{{My beautiful URL}}
to produce the URL.
DanB