[PHP7.4.11] 配列の足し算

$ary = [
	"いちご" => 1,
	"バナナ" => 2
];

foreach($ary as $key => $value){
	if($key == "いちご"){
		$ary["いちご"] = $value + 3;
	}
}
echo "<pre>";
var_dump($ary);
echo "</pre>";

なるほど、これを応用する

$ary = [
	"A商事" => 1200,
	"B商事" => 2000
];


$name ="C商事";
$subtotal = 2000;

foreach($ary as $key => $value){
	if($key == $name){
		$ary[$key] = $value + $subtotal;
	} else {
		$ary[$name] = $subtotal;
	}
}

echo "<pre>";
var_dump($ary);
echo "</pre>";

OK, これをLaravelでやりたい。