Laravel: Using $this when not in object context

New to object oriented programming so excuse me for this very basic question.

I'm working on a Laravel app where I've created a parent Graph class which has two subclasses.

In the parent class, I wanted to hold common functions that I want to use in the sub classes.

Graph:

<?php

namespace App\Graphs;

class Graph
{
    public function stringToFloat(?string $string): float
    {
        if (!$string) {
            return 0;
        }

        return \preg_replace('/[^0-9.-]+/', '', $string);
    }

    public function concatCosts(array $costs, string $table): string
    {
        $concatCosts = '';
        if (\count($costs)) {
            $concatCosts = \array_reduce($costs, function ($carry, $item) use ($table) {
                return $carry . ($carry ? '+' : '') . ($table . '.' . $item);
            });
        }

        return '+' . $concatCosts;
    }

    public function generateColor()
    {
        return '#' . \str_pad(\dechex(\mt_rand(0, 0xFFFFFF)), 6, '0', \STR_PAD_LEFT);
    }
}

Now using these methods like this fails:

<?php

namespace App\Graphs;

use App\User;
use Illuminate\Support\Collection;
use Illuminate\Support\Facades\DB;

class OrdersGraphs extends Graph
{
    public function totalSpendingOverTime(Collection $orders, array $suppliers, string $start, string $end): array
    {
        $data = DB::table('ordered_items')
            ->whereBetween('created_at', [$start, $end])
            ->whereIn('order_id', $orders)
            ->whereIn('supplier', $suppliers)
            ->select(
                [
                    'created_at',
                    DB::raw('SUM(price) as cogs'),
                ]
            )
            ->orderBy('created_at')
            ->groupBy('created_at')
            ->get();
        $total = DB::table('ordered_items')->select([DB::raw('SUM(price)')])->get();
        $totalInData = $data->reduce(function ($carry, $item) {
            return $carry + $this->stringToFloat($item->cogs);

The last line you see above fails and returns the error:

message: "Using $this when not in object context"

I do have the code running by using the static keyword next to the common functions and calling them like Graph:: stringToFloat() but it almost feels wrong. Shouldn't I be able to use $this here?

Thanks,