Fake marriages are marriages in which both parties do not mean an actual marriage which was set down by Allah SWA and the messenger ﷺ. Thus, they do not follow its conditions or pillars, and they are not keen on preventing its obstacles. In fact both parties agree upon having no sexual intercourse explicitly or implicitly. It is nothing more than an administrative procedure to gain some interest or remove harm. It is more like Tahleel marriage (marriage with deceit), where the marriage is not an actual marriage, but simply to allow the woman to become permissible to her husband who has divorced her three times.
A fake marriage is forbidden from a religious perspective, due to the wrong intentions behind it, and because this contract goes beyond the legitimate purposes, and contains contradictory conditions for its purpose. Therefore, it is forbidden for anyone who believes in Allah and the last day to take part in something like this.
As for the ruling on this, then it depends on how long the façade can be held in front of the judge. If the two parties acknowledge the falsity of the contract or the judge is certain of that through the circumstances and evidences he has collected, he decides to nullify it. If it is not proven, then it is judged to be valid as the pillars of marriage are fulfilled and its prohibitions are eliminated.
If there is a need to acquire some interests that can only be achieved through marriage, then the way to that is real marriage, which requires proper intention, fulfills the pillars and conditions of the marriage, and avoids the things it prohibits, and it is done according to the pure islamic law, where the timing is not stated in it nor has anyone tampered with the goals or objectives.