Filter Standard Deviation using PHP

I want to filter how many level I will add with my standard deviation result. These are steps that I need to do:

1. Calculate the standard deviation based on array value. (solve)

2. Find the average of the array value and add with the standard deviation result. eg : average = -49.7 s.d = 3.37

So, i need to keep adding the value until I get a new list of number. eg: -49.7, -46.33, -42.96, -39.59

After that, I need to filter only print the array value from -49.7 to -39.59 only.

Can anybody help me on this?

Here is the script from what I could understand the question.

If you need any explanation or the code is not what you want then let me know in the comment section.

<?php 

$array = [2,4,6,8,10];

$resultList = "";
$resultList_rounded = "";

function standardDeviation($array){

    //sum of all values
    $sum = array_sum($array);

    //sum of (square of values)
    $sum_of_square = array_sum(array_map(function($value){return $value*$value;},$array));

    //X Bar
    $xBar = average($array);

    //variance
    $variance = ($sum_of_square/count($array))-pow($xBar,2);

    //standard deviation
    return sqrt($variance);
}

function average($array){
    return (array_sum($array)/count($array));
}

function newList($array,$rounded=false,$round_digits = 4){
    $newarray = [];
    $sd = standardDeviation($array);
    $avg = average($array);

    for($i=1;$i<=count($array);$i++){
        if(empty($newarray)){
            array_push($newarray,$avg);
            continue;
        }
        if(!$rounded){
        array_push($newarray,$array[$i-1]+$sd);
        }else{
        array_push($newarray,number_format((float) ($array[$i-1]+$sd), $round_digits, '.', ''));
        }
    }
    return implode(',',$newarray);
}

function getRange($array){
    return $array[0]." to ".$array[count($array)-1];
}

//get new list
$resultList = newList($array,true,0); 
/*In the line above this, replace false with true if you want the rounded version and false if you want non rounded version. 4 means the number of digits to round.
examples:
$resultList = newList($array,true,4); will print 6,6.8284,8.8284,10.8284,12.8284
$resultList = newList($array,false,4);  will print 6,6.8284271247462,8.8284271247462,10.828427124746,12.828427124746
$resultList = newList($array,true,0); will print 6,7,9,11,13
$resultList = newList($array,true,1); will print 6,6.8,8.8,10.8,12.8
*/

//print the new result list
echo $resultList;