Well, 0.13 is equivalent to 0.130. The trailing zero is totally meaningless when it comes to identifying the value of the number. Just like 0000001 is just 1.
If you're trying to print the value of various numbers using a specific width/decimal place format, then you're talking about how that number is converted to a string, which is totally different.
You'll need to use the formatted output functions. printf() outputs your results directly, sprintf() returns the string so you can manipulate it or output it later.
$number = .13;
printf("The number is %01.3f.", $number); // outputs "The number is 0.130."
If you'd like to convert an array of numbers, look at vsprintf (vector sprintf)