Pass variables from php to javascript

rolwong

I am writing a web site for this purpose, but have no idea how to do this: can someone give me some hints or an example?

Let say, there are 5 classes, and 3 students in each class. All data are saved retrieved from DB. There is a drop down menu for the user to select a class. After the selection, three checkbox with student names of that class would show at the bottom. I was thinking to write this with PHP, but the problem is: after I select the drop down menu, I don't want to reload the php file to redraw the checkbox and the names. Then I was thinking to write with PHP and javascript. But I am new in both. Can someone tell me after I retrieved the data from DB, how can I pass them to javascript and ask javascript to draw the checkbox items?

Also if I need to save which checkbox has been checked in each class before I press the submit button, is it possible to do this?

Thanks for reading this first.

paulnaj

PHP can only do something at the server end ... it cannot react directly to a user action.

So unless you pull ALL the data you need to describe your page every time the user goes to this page, you will need to go back and forth to the server to update sections of the page.

If there ISN'T much data to extract, then doing it all in one go is feasible, but the general solution would be to go back to the server for updates.

rolwong

Because I am new in PHP, I am not sure if my data is huge or not. Can say I will have about 500-800 users. Every time i will retrieve user's name, class number and one more field. I would like to know if this can be counted as a heavy traffic?

I would also like to know, if data traffic is lesser. After I retrieve data from DB, i put it into $name[]. how can I transfer the data to javascript so that it can react user's side action?

paulnaj

1) Ok, what I really mean is, is the amount of data fixed or might it increase as more and more users use it. If it's the latter, then you really should do it by going back and forth to the server.

2) You can receive data back from the database in a few different formats and do lots of things with it, so let me know if this makes sense to you first ...

$result = mysql_query("SELECT id, name FROM mytable");
if(!$result) die("Cannot access database at this time.");

while($row = mysql_fetch_row($result)){
   echo "<hr>ID: ".$row["id"].": ".$row["name"];
}

rolwong

1) If it process in server side, it comes up a problem: eg. After user make the second selection, it needs to pass data to server side, back and forth, then it will reload the php file to redraw the table in order to show the student names of the selected class. The second problem is: I'd like to remember which checkbox the user has selected for different class. If the user has checked certain box for class1A, then reload it to draw the table of class1B. The selection of class1A's student may lose.

(2) It does make sense to me. The data is now PHP variable. How do I make javascript can use these data so that I can process some function similar to "onChange" a drop down menu?

sarahk

you may want to read up on sessions both here at at http://www.php.net
I would save the changes a user makes either back to the database or to a session.

I have an example of refreshing a page using iframes, so that the main page doesn't appear to submit see: http://property.digitalrice.com/muck/example.php

however the coding for this is probably more advanced than you should be tackling at this stage.

The onchange event that you talk about is still needed. The only way PHP will be run is because you request a page or submit a form. Javascript will run when clicks or changes are made to a page.