Okay well then you need to tell the script what to use as the information to store in the variable $PatientID. So what method are you using to pass the PatientID to your update.php script? In other words, what form method do you have defined for the form in which the user enters the id of the patient to update?
Should be something like (very basic form):
<form action="update.php" method="post">
<input type="text" name="Patient_ID">
<input type="submit" name="submit" value="Submit">
</form>
Now you should tell update.php to pull the Patient_ID from the url passed to it when the user clicks on the submit button. Put all this above your <html> tag btw.
<?
include("dbinfo.inc.php");
$Patient_ID = $_POST['Patient_ID'];
//This should be your query
mysql_connect(localhost,$username,$password);
@mysql_select_db($database) or die("Unable to Connect to Database");
$query = "SELECT * FROM patient WHERE Patient_ID=".$Patient_ID."";
$result = mysql_query($query);
$num = mysql_num_rows($result);
$patient = mysql_fetch_assoc($result);
?>
This should help better. Also keep in mind that if this is the update page you'll want to have a form outputted that has the values of the db fill in the values of the form so that the user can update the entries. Like the following:
<form action="updateSave.php" method="POST">
<table align="center" border="1" cellpadding="1" cellspacing="1" width="900">
<tr>
<td>Patient Name</td>
</tr>
<?
$Patient_First_Name = $patient['Patient_First_Name'];
while($num > 0){
echo "<tr>";
echo "<td><input type='text' name='Patient_First_Name' value='".$Patient_First_Name."'></td>";
echo "</tr>";
}
?>
Close the table.
</table>
Hope this helps!