You can't really create a pseudonym, but you might be able to detect if it is an iPhone and hide that button, otherwise hide the other one.
if((navigator.userAgent.match(/iPhone/i)) || (navigator.userAgent.match(/iPod/i)))
// PHP - server-side
if(strstr($_SERVER['HTTP_USER_AGENT'],'iPhone') || strstr($_SERVER['HTTP_USER_AGENT'],'iPod'))
This way both buttons would still be present to post data, but only one would be displayed to the user.
I don't know why this code I found uses match() and strstr() when they could use indexOf and strpos respectively.
The same code would work for an iPad, iPuddle, iPoodle, etc..