simple error.. but I can't solve it

[cmarti]

I'm creating previous/next links for query results.
I have an error about half way down..(see orange comment line)
I have seven entries in the database for testing purposes; the correct number is being displayed per page (4), however it says I have an invalid page number when I click next to view the other 3 entries and it doesn't display them.

Help will be greatly appreciated

<?php
// Number of entries per page 
$per_page = 4; 

$username = $_POST["StuUserID"];
$password = $_POST["StudPassword"];
$sql_text = ("select * from User_Info, GradeBook where User_Info.stuID=GradeBook.stuID AND User_Info.StuID='$username'and User_Info.Password='$password' ORDER BY DatePosted DESC"); 


// Set page #, if no page isspecified, assume page 1 
if (!$page) { 
   $page = 1; 
} 
$prev_page = $page - 1; 
$next_page = $page + 1; 

$query = mysql_query($sql_text);

// Set up specified page 
$page_start = ($per_page * $page) - $per_page; 
$num_rows = mysql_num_rows($query); 

if ($num_rows <= $per_page) { 
   $num_pages = 1; 
} else if (($num_rows % $per_page) == 0) { 
   $num_pages = ($num_rows / $per_page); 
} else { 
   $num_pages = ($num_rows / $per_page) + 1; 
} 
$num_pages = (int) $num_pages; 

if (($page > $num_pages) || ($page < 0)) { 
  // ERROR COMING FROM HERE
   echo "You have specified an invalid page number"; 
} 

// perform the actual displaying... 
$sql_text = $sql_text . " LIMIT $page_start, $per_page"; 
$query = mysql_query($sql_text); 

$found = false;
while ($result = mysql_fetch_array($query)) { 
    $found = true; 
    $username = $result[StuID]; 
    $password = $result[Password]; 
    $fname = $result[FirstName]; 
    $lname = $result[LastName]; 
    $cname = $result[ClassName]; 
    $glevel = $result[GradeLevel]; 
    $number = $result[Number];
    $assign = $result[Assignment]; 
    $grade = $result[Grade]; 
    $comments = $result[Comments]; 
    $dposted = $result[DatePosted]; 

echo " 
<div align=center>
  <center>
  <table width=90%>
    <tr>
      <td width=15% align=center>$cname</td>
      <td width=10% align=center>$number</td>
      <td width=20% align=center>$assign</td>
      <td width=10% align=center>$grade</td>
      <td width=15% align=center>$dposted</td>
      <td width=20% align=center>$comments</td>
    </tr>
  </table>
  </center>
</div>
"; 
} 

// Previous 
if ($prev_page)  {
   echo "<a href=\"$PHP_SELF?page=$prev_page\">Prev</a>"; 
}

// Page # direct links 
for ($i = 1; $i <= $num_pages; $i++) { 
   if ($i != $page) { 
      echo " <a href=\"$PHP_SELF?page=$i\">$i</a> "; 
   } else { 
      echo " $i "; 
   } 
}

// Next 
if ($page != $num_pages) { 
   echo "<a href=\"$PHP_SELF?page=$next_page\">Next</a>"; 
}
?> 

[Weedpacket]

Try a bit of debugging instrumentation:

echo "You have specified an invalid page number: 0 < $page < $num_pages is false.";