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11 years ago
Here's an implementation that adheres to the PECL source quite strictly. This is useful for people who want to use it but don't have the extension, as well as for people who want to understand what is going on.

<?php

if (!function_exists('stats_standard_deviation')) {
    /**
     * This user-land implementation follows the implementation quite strictly;
     * it does not attempt to improve the code or algorithm in any way. It will
     * raise a warning if you have fewer than 2 values in your array, just like
     * the extension does (although as an E_USER_WARNING, not E_WARNING).
     * 
     * @param array $a 
     * @param bool $sample [optional] Defaults to false
     * @return float|bool The standard deviation or false on error.
     */
    function stats_standard_deviation(array $a, $sample = false) {
        $n = count($a);
        if ($n === 0) {
            trigger_error("The array has zero elements", E_USER_WARNING);
            return false;
        }
        if ($sample && $n === 1) {
            trigger_error("The array has only 1 element", E_USER_WARNING);
            return false;
        }
        $mean = array_sum($a) / $n;
        $carry = 0.0;
        foreach ($a as $val) {
            $d = ((double) $val) - $mean;
            $carry += $d * $d;
        };
        if ($sample) {
           --$n;
        }
        return sqrt($carry / $n);
    }
}

?>
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