Correlation over 2 arrays

my_correlation.php

<?php
function my_correlation($a_x,$a_y){

    $len = count($a_x);

    $jml_x = 0;

    $jml_y = 0;

    $jml_xy = 0;

    $jml_xx = 0;

    $jml_yy = 0;

    $a_xy = array();

    $a_xx = array();

    $a_yy = array();

    for($i = 0; $i<$len; $i++){

        $jml_x = $jml_x + $a_x[$i];

        $jml_y = $jml_y + $a_y[$i];

        $a_xy[$i] = $a_x[$i] * $a_y[$i];

        $jml_xy = $jml_xy + $a_xy[$i];

        $a_xx[$i] = $a_x[$i] * $a_x[$i];

        $jml_xx = $jml_xx + $a_xx[$i];

        $a_yy[$i] = $a_y[$i] * $a_y[$i];

        $jml_yy = $jml_yy + $a_yy[$i];

    }


    $n = sqrt((($len * $jml_xx) - ($jml_x * $jml_x)) * (($len * $jml_yy) - ($jml_y * $jml_y)));

    $cor = (($len * $jml_xy) - (($jml_x) * ($jml_y))) / $n;

    return $cor;

}