Your question is rather unclear - do you mean to teleport them higher, or to change the player's physical height, such as make him taller?
Would it not be a floating point number? $player->setScale(.5); Think of it as multiplying, if you use 2, it multiplies by 2, or doubles the scale. If you multiply by 0.5, it divides the scale by 2, so it's half the size.
I don't think `.5` is a valid float literal syntax in PHP. And I have a question for everyone here: While you are teasing others for saying -1, have you really defined what "scale" means? Scale can be the scale of many things, from the linear scale of height to the quadratic scale of display area to the cubic scale of volume, or exponential scale or logarithmic scale. If it is exponential scale, -1 is very reasonable (although 0 would have to be the normal value).. According to Mojang's characteristic unpredictability [1], all kinds of scales would be reasonable [1]: How IP addresses were encoded I'm not saying that it's not linear scale. I just want to remind you to think twice before taking assumption as fact.
I believe it's just a multiplicative scale, in which case a negative number would get you a negative "length," though that makes sense.
This is exactly what I'm saying. You "believe" without evidence, which is primarily assumption. This is a very undesirable attitude when working with programs, especially when it has something to do with Mojang.