Store in a variable a data of the option marked in a select to then show it dynamically

<select name="p1" id="select1" required />
    <option selected="true" disabled="disabled">Elige un producto</option>
    <?php foreach($productos as $prod): ?>
    <option precio='<?php echo $prod['id'] ?>' value='<?php echo $prod['id'] ?>'><?php echo $prod['nombre']; ?></option>;
    <?php endforeach; ?>
</select>

I would do it that way it's cleaner, with jquery ($ ("# select1"). attr ('price');) you can calculate the price and save it in another variable within the field.

Also what you can do is process it with PHP. Ask your BBDD for the ID and calculate the price.

Using the JSON extension and following the idea of how you have the code, the following occurs to me:

<select name="p1" id="select1" required />
	<option selected="true" disabled="disabled">Elige un producto</option>
	<?php   
		foreach($productos as $prod)
		{
			echo "<option value='{".htmlspecialchars('"'.$prod['id'].'"', ENT_QUOTES, 'UTF-8').":".htmlspecialchars('"'.$prod['precio'].'"', ENT_QUOTES, 'UTF-8')."}'> ".$prod['nombre']." ( ".$prod['precio']." €) </option>";
		}
	?>
</select>

The previous code leaves each "value" in the following way:

"{"id_producto":"precio_producto"}"

Then using the extension JSON and its function json_decode (the product id as well as the price must be enclosed in double quotes in order to use json_decode):

json_decode($_POST['p1'], true)

This what it does is to return an associative array of the form:

[id_producto] => precio_producto

With this you obtain the ID and price of the product of the selection in a relatively simple way. I hope you serve, greetings.

Thanks for the answers. In the end I did it with jquery, something similar to what @Subiendo suggests.

<select name="p1" id="select1" required />
<option precio="0" selected="true" disabled="disabled">Elige un producto</option>
<?php foreach($productos as $prod): ?>
<option class="opcion" precio='<?php echo $prod['precio'] ?>' value='<?php echo $prod['id'] ?>'><?php echo $prod['nombre']; ?></option>;
<?php endforeach; ?>

But I added a class to the options of the select so that when I clicked one it tells me the price.

The js would look like this:

$('.opcion').on('click', function(){
var $precioSeleccionado = $(this).attr('precio');
});