numberHelper.php

<?php namespace JB;
/**
 * Class NumberHelper Test
 * @package JB
 * @author Jarrett Barnett <hello@jarrettbarnett.com>
 */
class NumberHelper
{
    /**
     * Get Sum of Values Squared By Array Key
     *
     * @param array $data
     * @param string $key
     * @return bool|number
     */
    public function getSumSquaresOfDataForKey($data = array(), $key = '')
    {
        if (!is_array($data))
        {
            return false;
        }

        // array_column requires PHP 5.5+
        $squares = $this->getSquareOfValues(array_column($data, $key));
        $sums = $this->getSumOfValues($squares);

        return $sums;
    }

    /**
     * Get Sum of Values
     *
     * @param null $input|mixed - returns the sum of an array containing only numbers
     * @return bool|number - false on invalid input, sum of values on success
     */
    public function getSumOfValues($input = NULL)
    {
        // check for arrays
        if (!is_array($input))
        {
            // we can only sum numbers/integers
            if (!is_numeric($input))
            {
                return false;
            }

            // convert value to simple array
            $input = [$input];
        }

        return array_sum($input);
    }

    /**
     * Get Square of Values
     *
     * @param null $input
     * @return array|bool
     */
    public function getSquareOfValues($input = NULL)
    {
        $result = [];

        // process input
        foreach ($input as $value)
        {
            // only process numbers
            if (!is_numeric($value))
            {
                continue;
            }

            $result[] = pow($value, 2);
        }

        return $result;
    }
}


// use our number helper class
$data = array(
    array('id' => 5, 'amount' => 10),
    array('id' => 6, 'amount' => 12),
    array('id' => 7, 'amount' => 4),
    array('id' => 8, 'amount' => 'apple'), // ignore this
    array('id' => 9) // ignore this
);

$numberHelper = new NumberHelper();
echo $numberHelper->getSumSquaresOfDataForKey($data, 'amount');