how to get currency to print correctly

[cutups]

i have a series of variables that have currency values in them

example:
$a=10.00
$b=5.40
$c=3.10

i add them together using an expression like

$d=$a+$b+$c;

and

echo $d;

ends up printing out 18.5

how can i get it to echo 18.50
?

[driverdave]

number_format