How to obtain value of a variable in another page [closed]

It is quite simple to receive the value of the product id in your file receive_products.php. look at this example

//obtienes el valor del id que estas enviando con get 

$_GET["id_product"];
//incluso puedes guardar ese valor en una variable para mas facilidad y reutilisamiento ejem:

$id_producto=$_GET["id_product"];

then you can do what you need with the value of that id for example make comparisons with conditions etc ..

I have created two URLs in localhost to implement your case.

link

URL 1 contains your link to ref

With the following code:

¡Hardcoding Values! 
<?php
    $row["id_product"] = 233;
    $row["product_name"] = "Curso de PHP con Laravel.";

<li><a href="recibir_productos.php?id=<?php echo$row["id_product"]?>"><?php echo$row["product_name"]?></a></li>

And URL 2:

link

shows the variable id by GET

Receiving parameters

<?php
 echo '¡Hola chosen, se recibió por GET el id '. $_GET["id"] .'!';

Do not forget that if in your URL the parameter is id=XXX , then in the PHP code you should catch the value así ===> $_GET['id'] ,

 ...?XXPARAM=xValue =  $_GET['XXPARAM'];

My version of php is 7.2.4

I would try require if it fails it produces the error (E_COMPILE_ERROR) and for the script, include only plays an alert (E_WARNING) but the script could be that it does not you are loading the script (conexion.php) and you do not realize it.

Another detail that I see is that you could be making the call but it does not connect (configuration error), try putting a confirmation code to the connection to the database, so you'll know if I connect, you could use a code like this later Connection Script (connection.php):

<?php

// connect to the database

$ mysql_server="localhost";

$ mysql_user="dbuser";

$ mysql_contra="password";

// Conectando, seleccionando la base de datos

$ link = mysql_connect ('mysql_server', 'mysql_user', 'mysql_contra')     or die ('Could not connect:'. mysql_error ());

echo 'successful connection to the database';

? >

if you can even select the table in the base so you do not have connection problems, I hope you serve, you comment!