Seems pretty simple: extract all pid values into their own array, run it through array_unique:

$uniquePids = array_unique(array_map(function ($i) { return $i['pid']; }, $holder));

The same thing in longhand:

$pids = array();
foreach ($holder as $h) {
    $pids[] = $h['pid'];
}
$uniquePids = array_unique($pids);

array_unique is using string conversion before comparing the values to find the unique values:

Note: Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same. The first element will be used.

But an array will always convert to Array:

var_dump("Array" === (string) array());

You can solve this by specifying the SORT_REGULAR mode in the second parameter of array_unique:

$unique = array_unique($a, SORT_REGULAR);

Or, if that doesn’t work, by serializing the arrays before and unserializing it after calling array_unique to find the unique values:

$unique = array_map('unserialize', array_unique(array_map('serialize', $a)));

You can use array_column to convert the array into an associative array. Which will override duplicate values. Use array_values to convert the associative array into a simple array.

$arr = //Your array

$result = array_values(array_column( $arr , null, 'product_code' ));

print_r( $result );

This will result to:

Array
(
    [0] => Array
        (
            [product_code] => prod3
            [qty] => 1
        )

    [1] => Array
        (
            [product_code] => prod4
            [qty] => 7
        )

)

Doc: array_column(), array_values()

if $mainArray = array(2)... , try this code:

$mainString = "";
foreach($mainArray as $array){
  foreach($array as $row){
    $mainString .= $row;
  } $mainString .= ", ";
}

$tempArray = explode(", ", $mainString);

$finishArray = array_unique($tempArray);

The simplest may be too extract the array indexing it by product_ID since indexes are unique:

$result = array_column($array, null, 'product_ID');

you have to do below modification in foreach() and code after that:

foreach ($rfq_product_master_data_array as $rfq_data_array) 
{
    $product_id = $rfq_data_array['product_id'];

    $explodedArray = explode(',',$rfq_data_array['vendor_id']);
    foreach($explodedArray as $value){
        $vendor_id[$value] = $value;
    }
}
print_r($vendor_id); //so you can use it further directly.

//if you want comma separated string then

echo $uniquePids = implode(',',array_values($vendor_id));

Special Notes: it will give you comma separated string of all unique vendor_id in each case, like suppose if you have

Array
(
    [0] => 1,2,3,4
    [1] => 5
    [2] => 1,2,6,7
)

it will produce:- 1,2,3,4,5,6,7

$result = array_reduce($array, function (array $result, array $item) {
    return array_filter($result) + $item;
}, []);

This will probably do what you want (which is slightly unclear).

Explanation: It goes through each of your items one by one; it filters all empty values out of it, leaving only populated keys (array_filter); it then adds all keys which don't already exist (+) from the next item to it (read up on array_reduce). The end result should be an array with all non-empty keys from all the arrays merged into one, with the value being the first non-empty value encountered within the loop.