Concate Explanation : i have a question, please don't downvote it, i need clear explanation.

Take a look at the code

        <?php
    $user_id = $_GET['user_id'];                  
    include "../database.php"; 
    $query="SELECT name FROM user WHERE user_id='$user_id'";
    $result=mysqli_query ($connect, $query);
    
    while($data = mysqli_fetch_array ($result))
    {
        $name=$data['name'];
        echo"<tr><td>$name</td></tr>";
    }                   
                        
    ?>

When i change into this one, the code still working. .  

    {
         echo"<tr><td>".$data['name']."</td></tr>";
        } 

But, when i change into this one, it is not working. .
 

    {
             echo"<tr><td>$data['name']</td></tr>";
            } 

Why the " and . does really matter?