diferença de duas horas guardadas na base de dados

fiz assim: (mas n dá valor nenhum)

>

<?php

echo "<a href='index.php'>voltar</a><br />";

//echo "<a href='exportar.php'>exportar</a><br />";

//echo "<a href='exportar2.php'>exportar2</a><br />";

echo "<a href='exportar3.php'>exportar</a><br />";

 

 

//$id = $_SESSION['username']

 

$con = mysql_connect("localhost","root","");

if (!$con)

 {

 die('Could not connect: ' . mysql_error());

 }

 

mysql_select_db("my", $con);

 

$result = mysql_query("SELECT * FROM Persons");

//$result2 = mysql_query("SELECT DATEDIFF(entra, sai) FROM persons");

$entra = $row['entra'];

$sai = $row['sai'];

$datadif = echo datediff('$sai', '$entra');

 

echo "<table border='1'>

<tr>

<th>nome</th>

<th>dia</th>

<th>mes</th>

<th>ano</th>

<th>turno</th>

<th>entra</th>

<th>sai</th>

<th>horas diarias</th>

<th>horas mensais</th>

<th>horas anuais</th>

</tr>";

while($row = mysql_fetch_array($result))

{

 echo "<tr>";

 echo "<td>" . $row['nome'] . "</td>";

 echo "<td>" . $row['dia'] . "</td>";

 echo "<td>" . $row['mes'] . "</td>";

 echo "<td>" . $row['ano'] . "</td>";

 echo "<td>" . $row['turno'] . "</td>";

 echo "<td>" . $row['entra'] . "</td>";

 echo "<td>" . $row['sai'] . "</td>";

 

 **echo "<td>" . $datadif . "</td>";**

 echo "<td>" . $row['horasmensais'] . "</td>";

 echo "<td>" . $row['horasanuais'] . "</td>";

 

 echo "</tr>";

 }

echo "</table>";

 

mysql_close($con);

 

?>