How to merge values in foreach loop php

You can try something like this

//This is your old array that you describe in your first code sample
$old_array = array();

// This will be the new joined array
$new = array();

// This will hold the key-pairs for each array within your initial array
$temp = array();

// This will hold all the values of orderId
$orderId = array();

// Loop through each first-level array with the original array
foreach($old_array as $val) {
    //Loop through each second-level array
    foreach($val as $key => $value) {
        // Set the key-pair values in the $temp array
        $temp[$key] = $temp[$value];

        if($key == "orderId") {
            // Add the current orderId value to the orderId array
            array_push($orderId,$value);

            // Join all the orderId values into the $temp array
            $temp[$key] = join(",", $orderId);
        }
    }

}

//Push the final values to the new array to get a 2 dimensional array
array_push($new, $temp);

Note: I did not test any of the following code so it is very likely to not work at first.
This is also VERY bad array design and there are more likely easier and more practical solutions to this problem, but you will need to give more details on what you want to achieve

$original_array = array(); //this is the array you presented

$new_array = array(); //this is the output array

foreach($original_array as $arr) {

    foreach($arr as $key => $value) {

        if(array_key_exists($key, $new_array)) { //if you already assigned this key, just concat
             $new_array[0][$key] .= "," . $value;
        } else { //otherwise assign it to the new array
             $new_array[0][$key] = $value;
        }
    }
}

It will give you the desired result

$arrNew = array();
    $i = 0;
    foreach($multiDArray as $array)
    {
        foreach($array as $key=>$value)
        {
            if($i > 0)
            {
                if($arrNew[$key] != $value)
                {
                    $str = $arrNew[$key].','.$value;
                    $arrNew[$key] = $str;
                }
            }
            else
            {
                $arrNew[$key] = $value;
            }       
        }
        $i++;   
    }

    print_r($arrNew);

Result: 
Array
(
    [QuizId] => 173
    [QuizName] => Reprocessing Surgical Drapes and Gowns
    [totalexams] => 1
    [UserScore] => 8
    [MaxScore] => 20
    [passed] => 1
    [CategoryId] => 1
    [CategoryName] => CRCST
    [totalTimes] => 1
    [orderId] => 19,20
    [productId] => 1
)