How do I set a PHP variable to have the value of a jQuery variable?

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PHP

Ajax

MySQL

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I've got the following issue:
I've got a table with orders. On each row there is an Edit button. When I click it a popup opens and the order is loaded to be modified. The problem is that I cannot pass the right value to the SELECT query in php.

function fnOpenNormalDialogEdit(jQueryParam) {

    <?php

    $phpVar = 'to have the value of jQueryParam';
    $OrderEdit = $wpdb->get_row($wpdb->prepare("SELECT * FROM $EWD_OTP_orders_table_name WHERE Order_ID='%d'", $phpVar));

    .....

}

If I pass

$phpVar = 19;

for example, the right order is loaded and everything is ok. But I cannot find a way to set that real value....

$phpVar = 'to have the value of jQueryParam';

I don't want to reload the page, I tried ajax but couldn't do it. Please help

Thanks,
Didi