Concatenate in PHP [duplicate]

Good morning, I'm starting PHP programming and today we have sent a simple little exercise to practice. What asks us is that the two echo are exactly the same, but in the first echo of the variable $sql if I show '2DAW' with the single quotes and in the other echo in which I concatenate the variable $aula shows me without single quotes.

<?php
    echo "<h3>SIN VARIABLE</h3>";
    $sql = "select * <br>
            from alumnos <br>
            where clase = '2DAW'";
    echo $sql;
    echo '<br><br>';

    echo "<h3>CON VARIABLE</h3>";
    $aula = "2DAW";
    $sql = "select * <br>
            from alumnos <br>
            where clase = ".$aula;
    echo $sql;
    echo '<br><br>';
  ?>