PHP numbers and text in a string

I'm trying to get $randpros to be something like this 0.15 but i got a problem $randpros = 0.$randpri; wont work.

Here is the code I've tried:

$randpri = rand(5,15);
$randpros = 0.$randpri;

is there another way to add 0. infront of $randpros?