How to pass javascript array to php array using Ajax and Jquery

I'm trying to pass a Javascript array into a PHP array through jQuery and Ajax. The PHP script should then write the array into a MySQL database, with each individual entry in the array being assigned to a separate column in the database. This is the code I'm currently using (which doesn't work!)

Javascript:

The array is contained in a variable called 'modules'

   $.ajax({
        url: "newaccount.php",
        type: "POST",
        data: {modules: modules},
        success: function(){
         modules.length = 0;
         $("#result").html("Success");
         },
     });

PHP:

 $modules = $_POST['modules'];

 if(mysql_query ("INSERT INTO level3 (mod1, mod2, mod3, mod4) VALUES ('$modules')")) {
  echo "Successfully inserted";
 }
 else {
  echo "Insertion Failed";
 }

This is a simplified version - usually there are other values inserted into the database alongside this which are not contained in arrays. These seem to be working fine, but when I try to insert the array values nothing goes in - the database columns remain as 'null'

Please help! I've been banging my head against this brick wall all day!