PHP不做我指定的数学运算？

Why isn't php subtracting $ltimeout from $ltimein? It just returns $ltimein. Here's my code:

if (isset($_POST['submit'])) {
    $name = $_POST['name'];

$querytimein = mysql_query("
    SELECT timein 
    FROM studentInfo
    WHERE name = '$name' 
    ORDER BY time DESC
    LIMIT 1
    ")
    or die("Error querying database ".mysql_error());
    $querytimeout = mysql_query("
    SELECT timeout
    FROM studentInfo
    WHERE name = '$name' 
    ORDER BY time DESC
    LIMIT 1
    ")
    or die("Error querying database ".mysql_error());
while($minutestimein = mysql_fetch_array($querytimein)){
    $ltimein = $minutestimein['timein'];
    }
        while($minutestimeout = mysql_fetch_array($querytimeout)){
    $ltimeout = $minutestimeout['timeout'];
  }
  $timegone = $ltimein - $ltimeout;
  echo $timegone;
}

BTW I know I need to switch to mySQLi, I will do that before I publish my website.

Since you are retrieving only a single row use 

$ltimein = mysql_fetch_row($querytimein); 

$ltimeout = mysql_fetch_row($querytimeout);

And then you could find the difference like this:

if(isset($ltimein) && isset($ltimeout)){

  $timediff = strtotime($ltimein['timein'])- strtotime($ltimeout['timeout']);

}

you can also do the time difference in mysql query:

select UNIX_TIMESTAMP(timein) - UNIX_TIMESTAMP(timeout) as time_diff from studentInfo where '$name'     ORDER BY time DESC    LIMIT 1