Undefined Variable

[bomberman]

Why does php show undefined variable? I researched and found out that if the variables aren't set then they will likely show undefined variable. However, I used the isset function to check if the variable is undefined and if it is then set it to $varaible ="" . That didn't work and then later i tried $variable =  NULL. What should I do Can you please see the code and tell me what shall i do. Thanks a million 

<?php require_once("includes/connection.php")?>
<?php require_once("includes/function.php") ?> 
<?php require_once("includes/header.php") ?>
<?php
    if(isset($_GET['subj'])){
        $sel_subj = NULL;
        $sel_subj = $_GET['subj'];
   
    } elseif (isset($_GET['page'])){
        $sel_page = NULL; 
        $sel_page = $_GET['page'] ;
        
    } else {
        $sel_subj= NULL;
        $sel_page =NULL; 
    }
?>

    <table id = "structure" >
        <tr> 
            <td id = "navigation" >
                <ul class= "subjects" > 
            <?php
                $subject_set = get_all_subjects();
                while ($subject = mysql_fetch_array($subject_set)){ // <a href = "content.php?subj=1" >
                    if ($sel_subj == $subject["id"]){
                        echo "<li class = \"selected\" ";
                    }else{
                        echo "<li> ";
                    }
                    "<a href = \"content.php?subj=" . urlencode($subject["id"]) . "\">" . $subject["menu_name"]. "</a></li>" ;
                    
                    
                    $page_set = get_pages_for_subjects( $subject["id"] ) ; 
                    echo " <ul class = \"pages\"> ";
                    while ($page = mysql_fetch_array($page_set)){
                        echo "<li><a href = \"content.php?page=" . urlencode($page["id"]) . "\">" . $page["menu_name"] . "</a></li>" ; 
                    }
                    echo "</ul>" ; 
                }
            
            ?>
                </ul>
            </td>
            
            <td id= "page" >
                <h1> Main Area To Get Your Information </h1>
                <?php echo $sel_subj ; ?> <br/>
                <?php echo $sel_page ; ?> <br/> 
                
            </td>    
        </tr>
    </table>
<?php include ("includes/footer.php") ?> 

The variable im refering to is $sel_subj and $sel_page at the top of the code. Here is an attachment of the error 

[Jacques1]

That looks like a couple of C&P errors. In your if statement on top, you overwrite the same variable twice while not setting the other one at all.

[jazzman1]

My logic says to me,

<?php
    if(isset($_GET['subj'])){
        $sel_subj = NULL; // to $sel_page = NULL; 
        $sel_subj = $_GET['subj'];
   
    } elseif (isset($_GET['page'])){
        $sel_page = NULL;  //to $sel_subj = NULL;
        $sel_page = $_GET['page'] ;
        
    } else {
        $sel_subj= NULL;
        $sel_page =NULL; 
    }
?>