How to select one cell through the selected id. SQL - php

Question

I have a database table in which there are 2 columns. The first is the ID, the second is the price. When the user selects a product on the site, a product card is opened (this will be id). At the bottom there is a calculator with prices where you need to load the price from the second column. Question: how to make a request for a specific ID(for loading price from second column). Here's the code that I could do.This code displays all IDs and prices, but need only when you select the product and only for it. Any help. Thank you
<?php
ini_set('display_errors','On');
error_reporting('E_ALL');//error's show
$host = 'localhost'; // host name
$database = 'test_sql'; // database name
$user = 'root'; // user name
$pswd = ''; // password
$dbh = mysql_connect($host, $user, $pswd) or die("Could not to connect MySQL.");//connect to mySQL
mysql_select_db($database) or die("Could not to connect database.");
$query = "SELECT * FROM `oc_product`";//load from product table
$res = mysql_query($query);
$row = mysql_fetch_array($res);//array call
while($row = mysql_fetch_array($res)){
echo "ID: ".$row['product_id']."<br>";//output for each ID
echo "Цена: ".$row['price']."<br>";//output for each price
}
?>

Answer 1

when you select a product pass that id to the function and update your query like :
$dbh = mysql_connect($host, $user, $pswd) or die("Could not to connect MySQL.");//connect to mySQL
mysql_select_db($database) or die("Could not to connect database.");
$query = "SELECT * FROM `oc_product` WHERE 'product_id' = <id that you passed>";//load from product table
$res = mysql_query($query);
$row = mysql_fetch_array($res);