PHP implementation of Pearson's product-moment correlation coefficient

pearson_correlation_coefficient

<?php
// PHP implementation of Pearson's product-moment correlation coefficient
function corr($a, $b) {
    $sum_ab = 0;
    $sum_a = 0;
    $sum_b = 0;
    $sum_a_sqr = 0;
    $sum_b_sqr = 0;
    $n = min(array(count($a), count($b)));
    for ($i = 0; $i < $n; $i++) {
        if (!isset($a[$i]) || !isset($b[$i])) { continue; }
        $sum_ab += $a[$i] * $b[$i];
        $sum_a += $a[$i];
        $sum_b += $b[$i];
        $sum_a_sqr += pow($a[$i], 2);
        $sum_b_sqr += pow($b[$i], 2);
    }
    return ($sum_ab/$n - $sum_a/$n * $sum_b/$n) / (sqrt($sum_a_sqr/$n - pow($sum_a/$n, 2)) * sqrt($sum_b_sqr/$n - pow($sum_b/$n, 2)));
}

$a1 = array(0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 0);
$b1 = array(1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 0, 1);
echo corr($a1, $b1) . "\n<br/>";

$a2 = array(1, 1, 1, 1, 0, 0, 1, 1, 1, 0, 0, 1, 0, 1);
$b2 = array(1, 1, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 0, 0);
echo corr($a2, $b2) . "\n<br/>";

$a3 = array(1, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 0);
$b3 = array(1, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 0);
echo corr($a3, $b3) . "\n<br/>";