You need to use sprintf or printf.

  $unrounded=0.66666;
  $rounded=sprintf("%.2f",$a) #$rounded is rounded to 2 decimal places
+ and is equal to 0.67
  printf("$unrounded rounded to 3 decimal places is %.3f\n",$unrounded
+);
[download]

You might also want to take a look at floor and ceil