What are "$phongexponent" and "$phongboost" and how do you use them?

[Sniffy]

The title says it all.

[Marco Skoll]

$phongexponent determines the width of a phong highlight.
$phongboost is a multiplier for the brightness of a phong highlight.

Valve's Phong shaders are reasonably well documented on the Valve Developer Wiki, so I'd suggest reading there for a detailed explanation:

https://developer.valvesoftware.com/wiki/$phong
https://developer.valvesoftware.com/wiki/Phong_materials