Php checkbox is checked, then mysql update 1, and if not checked mysql update 0

You could use isset() to see if the checkbox has been checked.

if(isset($_POST['checkbox']))
{
    $update = 1;
}
else
{
    $update = 0;
}

Then add $update to your query. Hope this helps.

First!

Please, don't use mysql_* functions in new code. They are no longer maintained and are officially deprecated. See the red box? Learn about prepared statements instead, and use PDO or MySQLi - this article will help you decide which. If you choose PDO, here is a good tutorial.

Seriously, your code contains a serious vulnerability called SQL Injection. If you don't fix it, your website can and will be compromised very easily.

Unchecked checkboxes do not get submitted at all. So basically, your server won't see anything.

To take that into account, what you should do is to set it to 0 by default, unconditionally, and then, if the checkbox was checked, set it to 1.

Yes, it does require you to possible do 2 queries in a request, but it's the only way to ensure you get the result you need.

Try $_SESSION vars like:

UPDATE pages SET page_value = '0' WHERE user_id = {$_SESSION['user_id']}
UPDATE pages SET page_value = '1' WHERE user_id = {$_SESSION['user_id']} AND page_id = '$page'  

Use mysql_real_escape_string for more security:

$pages = mysql_real_escape_string(serialize($_POST['page_ids']));

foreach($pages as $page) {
   // action
}

Solve the task in the right solution:

  echo "<form method='POST' action=''><br>";
    $query = mysql_query("set names 'utf8'");
    $query = mysql_query("SELECT * FROM pages WHERE user_id = '$_SESSION[user_id]'  ") or die(mysql_error());
    if(mysql_num_rows($query) == 0) {
            echo "<tr><td colspan=\"3\">Nincs megjeleníthető Facebook oldalad! Kérlek jelentkezz be az FB Login menüpont alatt!</td></tr>";
        } else {
            echo "<u>Kérem válassza ki melyik oldalaira szeretne képet feltölteni!</u><br><br><br>";

            while($pagek = mysql_fetch_array($query)) {
                $name = $pagek['page_name'];
                $pageid = $pagek['page_id'];
                 echo '<input type="checkbox" name="page_ids[]" value="'.$pageid.'">'.$name."<br>"; 

                                            }           

            echo "<br><input type='submit' value='Adatok frissítése' name ='adat' class='button small radius center black'>";


            echo"</form>";




            if($_POST['adat']){
            mysql_query("UPDATE pages SET page_value = '0' WHERE user_id = '$_SESSION[user_id]' ");


            foreach($_POST['page_ids'] as $page){
                $pa = $page;

                    if ($pa){
                        mysql_query("UPDATE pages SET page_value = '1' WHERE user_id = '$_SESSION[user_id]'  AND page_id = '$page'  ");
                            }

                                                }   
                            }

                        }
                        }