页面加载时php获取值

Question

In my website I have a tag like this: <input name="centro-med" type="hidden" value="prova" /> . In the php code (written in the same page) I need to get the value of that tag and use it for a query that is executed when the page loads. The problem is that I can't find a way to get that variable without having to submit a form(and so reload the page). I need that the php has already that value in a variable when the page loads for the first time so that I can use it for the query. I basically need something like this:

<input name="centro-med" type="hidden" value="prova" />

<?php
  $centro = $_POST['centro-med'];
  global $wpdb;
  $risultati = $wpdb->get_results("SELECT * FROM wp_table where centro_medico = '$centro'",ARRAY_A);
?>

I tried looking for other answers but nothing seems to have what I need. I hope you can help me

EDIT: I need it in a wordpress page, and since wordpress doesn't allow you to write php in pages I use a plugin with which I can write php snippets. I use the same code for different pages, the only thing that changes is that value (prova). So if I could only change that value instead of creating a new snippet for each page I could save lot of time. I don't now if that's understandable but I have reasons to not write it just inside the php. There was no question like this answered. Anyway I read the answers and I think that the only way to do so is with Ajax.

Answer 1

In order for PHP code to read a variable from HTML, it must be posted to the server. The HTML code is running in a users browser and the only way to communicate with the server-side code (PHP) is to submit a form, or an ajax call, etc..

Answer 2

Is it only one input element named "centro-med" ? Try XPath if you want the initial value...

Something like:

$dom = new DOMDocument();
$dom->loadHTML('...');
$xp = new DOMXpath($dom);
$nodes = $xp->query('//input[@name="centro-med"]');
$node = $nodes->item(0);

$value = $node->getAttribute('value');

Of course that is the initial, default value when the page loads. This is not what the user has typed in the input box, as by the time the user types anything in the input box, PHP will have finished running.