Database query failedYou have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LIMIT 1' at line 1
El error parece venir de una de estas dos funciones que no está recibiendo la variable $subject_id:
Código PHP:
   function get_subject_by_id($subject_id) {
    global $connection;
    $query = "SELECT * ";
    $query.= "FROM SUBJECTS ";
    $query.= "WHERE ID = " . $subject_id ." ";
    $query.= "LIMIT 1 ";
    $result_set = mysql_query($query, $connection);
    confirm_query($result_set);
    // REMEMBER
    // if no rows are returned, fetch array return false.
    if ($subject = mysql_fetch_array($result_set)) {
        return $subject;
    } else {
        return NULL;
    }
}
 
 
function get_page_by_id($subject_id) {
    global $connection;
    $query = "SELECT * ";
    $query.= "FROM PAGES ";
    $query.= "WHERE ID = " . $subject_id . " ";
    $query.="LIMIT 1 ";
    $result_set = mysql_query($query, $connection);
    confirm_query($result_set);
    // REMEMBER
    // if no rows are returned, fetch array return false.
    if ($subject = mysql_fetch_array($result_set)) {
        return $subject;
    } else {
        return NULL;
    }
} 
    Aquí muestro el contenido de edit_subject.php
Código PHP:
   <?php require_once ("includes/connection.php");?>
<?php require_once ("includes/functions.php");?>
<?php 
    find_selected_page();    
?>
 
<?php
    if (intval($_GET['subj']) == 0 ) {
        redirect_to("content.php");
        
    }
    if (isset($_POST['submit'])) {
        $errors = array();
        
        $required_fields = array('menu_name', 'position', 'visible');
        foreach($required_fields as $fieldname) {
            if (!isset($_POST[$fieldname]) || (empty($_POST[$fieldname]) && ($_POST[$fieldname]) !=0)) {
                $errors[] = $fieldname;
            }
        }
        $fields_with_lenghts = array('menu_name' => 30);
        foreach($fields_with_lenghts as $fieldname => $maxlength) {
            if (strlen(trim(mysql_prep($_POST[$fieldname]))) > $maxlength) {
                $errors[] = $fieldname;
            }
        }
        
        if (empty($errors)) {
            //perform update
            $id = mysql_prep($_GET['subj']);
            $menu_name = mysql_prep($_POST['menu_name']);
            $position = mysql_prep($_POST['position']);
            $visible = mysql_prep($_POST['visible']);
            
            $query = "UPDATE subjects SET
                        menu_name = '$menu_name', 
                        position = $position,
                        visible = $visible
                      WHERE id = $id";
            $result = mysql_query($query, $connection);
            if (mysql_affected_rows() == 1) {
                //Success
            } else {
                //failed
            }
            
        } else {
            //errors occurred    
        }
        
        
        
        
    } // end if (isset($_POST['submit']))
?>
 
<?php include ("includes/header.php");?>
        <table id="structure">
            <tr>
                <td id="navigation">
                    <?php echo navigation($sel_subject, $sel_page);?>
                </td>
                <td id="page">
                <h2>Edit Subject <?php echo $sel_subject['menu_name'];?></h2>
                <form action="edit_subject.php?subj=<?php urlencode($sel_subject['id']); ?>" method="post">
                    <p>Subject Name:
                        <input type="text" name="menu_name" value="<?php echo $sel_subject['menu_name'];?>" id="menu_name" />
                        
                    </p>
                    <p>Position:
                        <select name="position">
                        <?php
                            //subject_count +1 because we're adding a subject
                            $subject_set = get_all_subjects();
                            $subject_count = mysql_num_rows($subject_set);
                            for ($count=1; $count <= $subject_count+1; $count++){
                                echo "<option value=\"{$count}\"";
                                if ($sel_subject['position'] == $count) {
                                    echo "selected";
                                }
                                echo ">{$count}</option>";
                                
                            }
                        ?>
                            
                        </select>
                    </p>
                    <p>Visible:
                    <input type="radio" name="visible" value="0"<?php
                    if ($sel_subject['visible'] == 0) {echo "checked";}?> /> No
                     
                    <input type="radio" name="visible" value="1"<?php
                    if ($sel_subject['visible'] == 1) {echo "checked";}?> /> Yes
                    </p>
                    <input type="submit" name="submit" value="Edit Subject" />
                 </form>
                <br />    
                <a href="content.php">Cancel</a>
                     
                </td>
             </tr>
          </table>
       </div>
<?php require ("includes/footer.php");?>     
 



 
 

