将php连接为html id

I am trying to concatenate $firstname and $lastname as a select box id. Where am I going wrong?

echo '<td>
<select name="'$firstname.$lastname'" id="'$firstname.$lastname'">
       <option selected disabled>Select Action</option>
   <option value="activate">Activate User</option>
       <option value="delete">Delete User</option>
   <option value="admin">Admin</option>
</select>
  </td>';

You need to add concatenators before and after the first and last name:

<select name="'.$firstname.$lastname.'" id="'.$firstname.$lastname.'">

Your concatenation is incorrect. Use the following:

echo '<td>
 <select name="'.$firstname.$lastname.'" id="'.$firstname.$lastname.'">
       <option selected disabled>Select Action</option>
   <option value="activate">Activate User</option>
       <option value="delete">Delete User</option>
   <option value="admin">Admin</option>
 </select>
</td>';