如何在每次单击按钮然后使用它进行查询时获取php变量？

I`am trying to get the $row['username'] every time I click on the button, for me to use it in the query . But only the first array is displayed. I want to use jquery but I have small knowledge on it.

<?php
  $query = $con->query("SELECT * FROM account WHERE acc = 'agency'");
  while($row = mysqli_fetch_array($query))
  {
    echo $row['username'];
  ?>
    <button data-target="#select" data-toggle="modal">Select</button>
<?php } ?>


  <div class="modal" id="select">
  <?php
    $query = $con->query("SELECT * FROM sample WHERE username = ".$row['username']."");

  ?>

<button id="select"
        class="selectUser"
        data-username="<?php echo $row['username']; ?>"
        data-toggle="modal"
        data-target="#select">Select</button>

I took the html you put in your comment, and added a class to it. Now all you need to do is create your javascript binding for it.

//find all the buttons with the selectUser class and bind a click handler to them
$('.selectUser').on('click', function(e){
    //turn the element into a jQuery object so we can use jQuery methods
    var $selectUser = $(this);
    //log the username data attribute from the button
    console.log($selectUser.data('username'));

    //you could also get the attribute without jQuery
    console.log(this.getAttribute('data-username'));
});