<?php
//the current array and an empty new array to get the result.
$cur_array = [0 => "4,9", 1 => "5,9"];
$new_array = [];

//run through all array values to convert these.
for ($i = 0; $i < count($cur_array); $i++) {

    //get an array of the comma separated list.
    $part = explode(',', $cur_array[$i]);

    //add the array of comma separated list to the new array (result).
    $new_array = array_merge($new_array, $part);
}

//convert all string values to integer values.
for ($j = 0; $j < count($new_array); $j++) {
    $new_array[$j] = intval($new_array[$j]);
}

//output of the new array (debug).
var_dump($new_array);