It's not explicitly documented; number_format also rounds:

<?php
$numbers = array(0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009);
foreach ($numbers as $number)
    print $number."->".number_format($number, 2, '.', ',')."<br>";
?>

0.001->0.00
0.002->0.00
0.003->0.00
0.004->0.00
0.005->0.01
0.006->0.01
0.007->0.01
0.008->0.01
0.009->0.01

Outputs a human readable number.

<?php
    #    Output easy-to-read numbers
    #    by james at bandit.co.nz
    function bd_nice_number($n) {
        // first strip any formatting;
        $n = (0+str_replace(",","",$n));
        
        // is this a number?
        if(!is_numeric($n)) return false;
        
        // now filter it;
        if($n>1000000000000) return round(($n/1000000000000),1).' trillion';
        else if($n>1000000000) return round(($n/1000000000),1).' billion';
        else if($n>1000000) return round(($n/1000000),1).' million';
        else if($n>1000) return round(($n/1000),1).' thousand';
        
        return number_format($n);
    }
?>

Outputs:

247,704,360 -> 247.7 million
866,965,260,000 -> 867 billion

Note: use NumberFormatter to convert in human-readable format instead  user function from comments:
<?php
echo NumberFormatter::create('en', NumberFormatter::SPELLOUT)->format(12309); // twelve thousand three hundred nine
echo NumberFormatter::create('ru', NumberFormatter::SPELLOUT)->format(12307.5); //  двенадцать тысяч триста семь целых пять десятых
?>

If you want to display a number ending with ,- (like 200,-) when there are no decimal characters and display the decimals when there are decimal characters i use:

function DisplayDouble($value)
  {
  list($whole, $decimals) = split ('[.,]', $value, 2);
  if (intval($decimals) > 0)
    return number_format($value,2,".",",");
  else
    return number_format($value,0,".",",") .",-";
  }

formatting numbers may be more easy if u use number_format function.

I also wrote this :
function something($number)
{
    $locale = localeconv();
    return number_format($number,
       $locale['frac_digits'],
        $locale['decimal_point'],
        $locale['thousands_sep']);
}

hope this helps =)
[]'s

For Zero fill - just use the sprintf() function

$pr_id = 1;
$pr_id = sprintf("%03d", $pr_id);
echo $pr_id;

//outputs 001
-----------------

$pr_id = 10;
$pr_id = sprintf("%03d", $pr_id);
echo $pr_id;

//outputs 010
-----------------

You can change %03d to %04d, etc.

I ran across an issue where I wanted to keep the entered precision of a real value, without arbitrarily rounding off what the user had submitted.

I figured it out with a quick explode on the number before formatting. I could then format either side of the decimal.

<?php
      function number_format_unlimited_precision($number,$decimal = '.')
      {
           $broken_number = explode($decimal,$number);
           return number_format($broken_number[0]).$decimal.$broken_number[1];
      }
?>