Insert array value into database

I have an array of checkboxes whose value I want to insert into a database. The checkbox declaration is like this:

echo '<td><input type="checkbox" name="check_list[]"       
onClick="toggle(this, '.$x.')" /> All</td>';

And here I try to insert it (together with other variables, that doesnt really matter). So, the other variables are inserted, while this one isnt.

<?php
    ob_start(); //eliminates buffer collisions
    require_once('connect_db.php'); 
    $id = time(); //creates a unique id using the unix time
    $result = pg_query(connect(), "INSERT INTO lecturer VALUES ($id, '$_POST[name]','$_POST[surname]','$_POST[department]')");             

     $check =  $_POST['check_list[0]'];         
     $availability = pg_query(connect(), "update lecturer set monday='$check'");    

?>

What is wrong with my syntax? Cause all the values are inserted, besides that of the checkbox.