How can i sum an array like this in laravel

You will need to use array_reduce for the special format you have

$investamount = DB::table('investment')->pluck('amount')->toArray();
$investamount = array_reduce($investamount, function($carry, $item) {
    return $carry + str_replace(',','',$item);
});

It seems you are using varchar for your amount field having comma then you can do it like this

DB::table('investment')->select("SUM(REPLACE(amount, ',', '')") as total)->get()

use

$investamount = DB::table('investment')->sum('amount');

or

$investamount = DB::table('investment')->pluck('amount')->toArray();
$sum = array_sum($investamount);

EDITED

$investamount = DB::table('investment')->select(DB::raw("SUM(amount) as sum"))->get();
return $investamount->sum;

Here is what you need

return DB::table('investment')->get()->sum(function ($investment) {
    return (float) str_replace(',', '', $investment->amount);
});

You can achieve that using laravel Collections. Then map the items to replace the commas.

Example:

return collect(DB::table('investment')->select('amount')->get())->map(function ($item) {
  return str_replace(',', '', $item->amount);
})->sum(); //This will return 425406‬.00

If you want to display the sum with commas in your blade template you can use number_format()

Example:

{{ number_format(sum, 2) }} //425,406‬.00