Thanks to the talk with Sarfraz we could figure out the solution.
The problem was that I was passing an HTML element instead of its value, which is actually what I wanted to do (in fact in my php code I need that value as a foreign key for querying my cities table and filter correct entries).
So, instead of:
var data = {
‘mode’: ‘filter_city’,
‘id_A’: e[e.selectedIndex]
};
it should be:
var data = {
‘mode’: ‘filter_city’,
‘id_A’: e[e.selectedIndex].value
};
Note: check Jason Kulatunga’s answer, it quotes JQuery doc to explain why passing an HTML element was causing troubles.
From the jQuery docs for processData:
processData Boolean
Default: true
By default, data passed in to the data option as an object (technically, anything other than a string) will be processed and transformed into a query string, fitting to the default content-type “application/x-www-form-urlencoded”. If you want to send a DOMDocument, or other non-processed data, set this option to false.
Source: http://api.jquery.com/jquery.ajax
Looks like you are going to have to use processData to send your data to the server, or modify your php script to support querystring encoded parameters.