Can you pass data from MySQL to PHP and then pass it to another PHP? (or in Js)

What you try to do is create the product detail, for this you have to send the id of the product to the page where it will show more detail of it.

We go for part, in the query you perform to show the list of products, you must call an additional data of the table in this case the id of the product, noted in your code that you are already using $row['productid']; .

So, now what is needed is to add to the query where you show the list of products as follows:

<a href="details.php?id=<?php echo $row['productid']; ?>">Más detalle</a>

Now if you click on [More detail] of a product, for example: detail.php?id=1 and then from PHP get the value of the URLs with $_GET

Example:

details.php

if (isset($_GET['id'])){
  $id = $_GET['id'];
  $sql = "SELECT * FROM product WHERE productid='".$id."'";
  $result = mysqli_query($conn, $sql);
}

if(mysqli_num_rows($result) > 0){
while($row = mysqli_fetch_array($result)){
    $id = $row['productid'];
    $title = $row['title'];
    $product = $row['product'];
    $precio = $row['price'];
    $imagen = $row['image'];        
  }
} else {
  echo "Lo sentimos no existe detalle de este producto por el momento";
}
?>

Then you can show what you want on the page detail.php or as you wish to call it, as follows: <h1><?php echo $product; ?></h1>

That would be all ...

Now I just have to give some recommendations, use prepared statements to avoid possible SQL injections, possible system vulnerabilities among others.

More information on the subject, read the following sources:

• What is SQL injection and how can I avoid it?
• How to avoid injection SQL in PHP?

Yes, one option would be to pass the id of your entity to the other page by means of the method GET by URL , then from PHP get the value with $_GET and make the respective query . (I assume that product_id is your unique key)

Current Page

 <div style="height: 10px;"></div>
 <div><a href="detalle.php?id=<?php echo $row['product_id']; ?>">Ver Más</a></div>

In your file detalle , you get the id of the element and make the respective query with its id and you give it the appropriate format.

detalle.php

<?php 

if(isset($_GET['id']) && !empty($_GET['id'])){
  $id = $_GET['id'];
  //Query y el resultado le da el formato y estilo que desee.
}
else{
    header('Location:index.php');
}

  

Regarding the file detalle.php, you must take into account at the moment   to perform the query does not concatenate the value. For more detail   review How to avoid SQL injection in PHP?

You can, another way is to use sessions Sessions in PHP create a document that stores the product

<?php
   session_start(); //inicia la sesión
   $_SESSION["productoid"] = $_POST["id"];//guardar el id
   //puede crear tantas variables de sesiones requieras aqui solo uso una
?>

using Jquery send the value to the previous document

$("button").click(function(){
     $.ajax({                        
           type: "POST",                 
           url: "documento_sesion.php", //el ducumento anterior                    
           data: {"id":"valor","otro":"valor"},  //el VALOR lo recuperas con Jquery por ejemplo $("#valor").val(); puedes enviar varios datos
           success: function(data)             
           {
            //guardado exitoso               
           }
       });
});

In the other PHP you only recover the value (or values) of the session:

<?php
   session_start(); //recupera la sesión
   $id=$_SESSION["productoid"];
?>