getElemntbyid returns undefined value

What's the point in getting elements by id if the id's are dynamic?
Instead, use a css class as the selector. Then, use the $(this).parents('tr') method to get the row in the table.

Example:

PHP Code:

<table>
  <?php foreach($rows as $row) : ?>
      <tr>
        <td><input type="text" name="text_one[]" class="findrow" /></td>
        <td><span class="showme"><?= $row['id'];?></span></td>
     </tr>
  <?php endforeach; ?>
</table> 


Code:

<script>
$(document).ready(function(){
  $('.findrow').change(function(){
     row = $(this).parents('tr');
     t2 = row.find('.showme').html();
     alert(t2);
  });
});
</script>

After you change the value in one of the input fields, the id of the current row will be displayed in an alert box.