Calculate covarience in php

Covarience Calculation function in PHP

This is the function to calculate the covarience in php. There is also stats library that has inbuilt this function.

function custom_covarience($aValues,$bValues){  //array variables of same numbers
$mean_a = array_sum($aValues)/count($aValues);
$mean_b = array_sum($bValues)/count($bValues);
$ret = array();
$sum = 0;
for($i=0;$i<count($aValues);$i++)
{
$sum =  $sum + ($aValues[$i] - $mean_a) * ($bValues[$i] - $mean_b);
}
$covarience = $sum/count($aValues);
return $covarience;

}