counterpart of C#'s listBox1_SelectedIndexChanged in JS

Is the开发者_如何学JAVAre such a thing as listBox1_SelectedIndexChanged but in PHP ?

Nope, but you could use jQuery :)

$(document).ready(function() {
    $("#selectList").change(function() {
        var myVal = $(this).val();
        alert(myVal);
    });
});

If you need to send data to server and get back from it, use $.post() / $ajax() etc ...