case 'signature':
signature_change();
break;
<a href='preferences.php?action=signaturechange'>Prof ile Signature</a></br>
The link isn't working because the action you've put in the link doesn't match the case you've set of 'signature'.
Anyhow; You're not validating the data you're receiving *at all*, I can tell that you're not very experienced so I don't think you'll be able to implement this into your game without coming into any security implications with this.