- sqrt: documentation ( source)
<?php
$array = array(9,4,-3,-10);
function stddev($array){
//Don Knuth is the $deity of algorithms
//http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#III._On-line_algorithm
$n = 0;
$mean = 0;
$M2 = 0;
foreach($array as $x){
$n++;
$delta = $x - $mean;
$mean = $mean + $delta/$n;
$M2 = $M2 + $delta*($x - $mean);
}
$variance = $M2/($n - 1);
return sqrt($variance);
}
stddev($array);
?>