If you're having trouble enumerating the reasons why you'd use scala/lift over PHP, you'll probably want to stick with PHP. PHP is a far more mature and supported platform - oodles of documentation, huge community, tons of tools and libraries.
Scala is a great language, but the primary reason it's being used by Twitter and Foursquare is it's ability to work with enormous sets of data on a horizontally scaled architecture. These are very specific problems for these companies, and Scala was chosen for this purpose very deliberately.
As someone who has recently begun learning Scala, the community and documentation is minute compared to PHP's. Especially where Lift is concerned. And things like IDE support is still in it's early stages.
There's really no way to compare the two technologies (very different approaches). If want to use Scala, use it, but you'll probably be investing more time and energy if you're not already proficient (also, some background in Java doesn't hurt). PHP is just PHP, it's so easy to just pick up and use.