echo语句中的Echo Variable

I am trying to echo a php variable inside of a php echo statement.

I have a column in a MySql database called display_code which contains a value such as $num_rows_customers.

I am calling this from the DB as displayCode

$displayCode = $row ['display_code'];

And trying to echo like this:

 <?php    
           echo "<div class='col-md-3'>
           $displayCode
           </div>";

 ?>

However this is printing $num_rows_customers instead of a number

Is there a way to do this?

By the way, I already have this script on the page

<?php $result100=mysql_query("SELECT * FROM customers")or die('Error 27' . mysql_error());
$num_rows_customers = mysql_num_rows($result100);?>

About Variable variables read here

$displayCode = 'num_rows_customers';
$num_rows_customers = 10;  
           echo "<div class='col-md-3'>
           {$$displayCode}
           </div>";

demo