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