round

In my opinion this function lacks two flags:
- PHP_ROUND_UP - Always round up.
- PHP_ROUND_DOWN - Always round down.
In accounting, it's often necessary to always round up, or down to a precision of thousandths.
<?php
function round_up($number, $precision = 2)
{
 $fig = (int) str_pad('1', $precision, '0');
 return (ceil($number * $fig) / $fig);
}
function round_down($number, $precision = 2)
{
 $fig = (int) str_pad('1', $precision, '0');
 return (floor($number * $fig) / $fig);
}
?>

As PHP doesn't have a a native number truncate function, this is my solution - a function that can be usefull if you need truncate instead round a number.
<?php
/**
 * Truncate a float number, example: <code>truncate(-1.49999, 2); // returns -1.49
 * truncate(.49999, 3); // returns 0.499
 * </code>
 * @param float $val Float number to be truncate
 * @param int f Number of precision
 * @return float
 */
function truncate($val, $f="0")
{
 if(($p = strpos($val, '.')) !== false) {
 $val = floatval(substr($val, 0, $p + 1 + $f));
 }
 return $val;
}
?>

Originally posted in http://stackoverflow.com/a/12710283/1596489 

If you have negative zero and you need return positive number simple add +0:
$number = -2.38419e-07;
var_dump(round($number,1));//float(-0)
var_dump(round($number,1) + 0);//float(0)

I discovered that under some conditions you can get rounding errors with round when converting the number to a string afterwards.
To fix this I swapped round() for number_format().
Unfortunately i cant give an example (because the number cant be represented as a string !)
essentially I had round(0.688888889,2);
which would stay as 0.68888889 when printed as a string.
But using number_format it correctly became 0.69.

function mround($val, $f=2, $d=6){
 return sprintf("%".$d.".".$f."f", $val);
}
echo mround(34.89999); //34.90

Unexpected result or misunderstanding (php v5.5.9)
<?php
echo round(1.55, 1, PHP_ROUND_HALF_DOWN); // 1.5
echo round(1.551, 1, PHP_ROUND_HALF_DOWN); //1.6
?>

If you'd only want to round for displaying variables (not for calculating on the rounded result) then you should use printf with the float:
<?php printf ("%6.2f",3.39532); ?>

This returns: 3.40 .

This function will let you round to an arbitrary non-zero number. Zero of course causes a division by zero.
<?php
function roundTo($number, $to){
 return round($number/$to, 0)* $to;
}
echo roundTo(87.23, 20); //80
echo roundTo(-87.23, 20); //-80
echo roundTo(87.23, .25); //87.25
echo roundTo(.23, .25); //.25
?>

Here is function that rounds to a specified increment, but always up. I had to use it for price adjustment that always went up to 5ドル increments.
<?php 
function roundUpTo($number, $increments) {
 $increments = 1 / $increments;
 return (ceil($number * $increments) / $increments);
}
?>

this function (as all mathematical operators) takes care of the setlocale setting, resulting in some weirdness when using the result where the english math notation is expected, as the printout of the result in a width: style attribute!
<?php
$a=3/4;
echo round($a, 2); // 0.75
setlocale(LC_ALL, 'it_IT@euro', 'it_IT', 'it');
$b=3/4;
echo round($b,2); // 0,75
?>

/**
 * Round to first significant digit
 * +N to +infinity
 * -N to -infinity
 *
 */
function round1stSignificant ( $N ) {
 if ( $N === 0 ) {
 return 0;
 }
 $x = floor ( log10 ( abs( $N ) ) );
 return ( $N > 0 )
 ? ceil( $N * pow ( 10, $x * -1 ) ) * pow( 10, $x )
 : floor( $N * pow ( 10, $x * -1 ) ) * pow( 10, $x );
}
echo round1stSignificant( 39144818 ) . PHP_EOL;
echo round1stSignificant( 124818 ) . PHP_EOL;
echo round1stSignificant( 0.07468 ) . PHP_EOL;
echo round1stSignificant( 0 ) . PHP_EOL;
echo round1stSignificant( -0.07468 ) . PHP_EOL;
/**
 * Output
 * 
 * 40000000
 * 200000
 * 0.08
 * 0
 * -0.08
 * 
 */

round() will sometimes return E notation when rounding a float when the amount is small enough - see https://bugs.php.net/bug.php?id=44223 . Apparently it's a feature.
To work around this "feature" when converting to a string, surround your round statement with an sprintf:
sprintf("%.10f", round( $amountToBeRounded, 10));

Here's a function to round to an arbitary number of significant digits. Don't confuse it with rounding to a negative precision - that counts back from the decimal point, this function counts forward from the Most Significant Digit.
ex:
<?php
round(1241757, -3); // 1242000
RoundSigDigs(1241757, 3); // 1240000
?>

Works on negative numbers too. $sigdigs should be >= 0
<?php
function RoundSigDigs($number, $sigdigs) {
 $multiplier = 1;
 while ($number < 0.1) {
 $number *= 10;
 $multiplier /= 10;
 }
 while ($number >= 1) {
 $number /= 10;
 $multiplier *= 10;
 }
 return round($number, $sigdigs) * $multiplier;
}
?>