How to execute while loop inside single quote string in php

I am a PHP beginner and working on a simple project for learning purpose and at this point am trying to execute a while loop inside another while loop. But the second loop is inside a single quote string. The outputs of both loops are stored in a normal html/bootstrap table cells. At first I was able to get the output of the first loop but the problem came in when I added the second loop inside the string. I have already researched about using quotations in PHP but I still can't figure out the problem. Below is the code. Someone help please.

  $output .='<div class="table-responsive">
  <table class="table table-bordered"><form>';
  

  while($row = $result->fetch_assoc()){
      $output .='


     <tr>
     <td width="30%"><label>Price</label></td>
     <td>
     <div class="form-group">
     <input type="text" class="form-control"  placeholder="'.$row['price'].'">
   </div>
   </td>
    </tr>

   <tr>
   <td width="30%"><label>Writer</label></td>
   <td>
   <div class="form-group">
   
     <select class="form-control" id="">
     
       '.while($col = $res->fetch_assoc()){.' 
     <option value="" disabled selected>'.$row['name'].'</option>
     <option value="" >'. $col['username'].'</option>
       '.} .'
     </select>
   
 </div>
 </td>
  </tr>
  

   ';
  }
  $output .="</table></div></form>";
  echo $output;
}

The error am getting is: Parse error: syntax error, unexpected 'while' (T_WHILE) in C:\xampp\htdocs\Writers_SM\select.php on line 61(the second while loop )

source https://stackoverflow.com/questions/70481934/how-to-execute-while-loop-inside-single-quote-string-in-php