Well, assuming you know some things about the number such as it is a 2 or three digit number that is going to only be used as the height of a human being between 1 foot 0 inches and 9 feet, + inches, this would be simple to do:
$height= 45;
$heighttxt = $height[0] . "\' ". $height[1] . $height[2] . "\"";
this should display: 4' 5"
Again, if you were trying to enter 12' 11", this would not work but that wouldn't be too much harder.
-Ben