How can I set the value of data-id in php to a variable?

Question

The following code works

echo '<td class="center"><a data-toggle="modal" class="delete" data-id="30" data-target="#myModal" href="#">Delete22</a></td>';

But the following code does not work

echo '<td class="center"><a data-toggle="modal" class="delete" data-id=$aa   data-target="#myModal" href="#">Delete22</a></td>';

Thanks friends