PHP Binance API Price Problem Auto Calculate

Question

I'm using PHP Binance API. I make a limit order succesfuly. For Example;

$quantity = 1500000;
$price = 0.00001000;
$order = $api->buy("SHIBUSDT", $quantity, $price);

But i don't want use quantity value. I want give a price and it must be calculate coin quantity. For example;

$price = 0.00001000;
$quantity = 100 BNB or USDT;

Thanks for your help.