PHP - Usar variable dentro de consulta

<table border="0" cellpadding="0" cellspacing="2">

    <tr>

        <td>Concepto</td>

        <td>Cantidad</td>

    </tr>

    <?php

    require("connect_db.php");

    $empresa = "Papeleria Lupita";

    $sql="SELECT * FROM '$empresa'";

    $result=mysqli_query($mysqli,$sql);

    while($mostrar=mysqli_fetch_array($result)){

     ?>

    <tr>

        <td><?php echo $mostrar['concepto'] ?>: </td>

        <td><?php echo $mostrar['cantidad'] ?></td>

    </tr>

<?php

}

?>

</table>

<?php

require("connect_db.php");

$empresa = "Papeleria Lupita";

$sql="SELECT * FROM '$empresa'";

$result=mysqli_query($mysqli,$sql);

if (!$result) {

    printf("Errormessage: %s\n", mysqli_error($mysqli));

}

$sql="SELECT * FROM '$empresa'";

$sql="SELECT * FROM $empresa";