if / else在php脚本中不能处理ajax请求

I'm working on a website with heavy Ajax, and I'm using codeigniter for building it.
I've got a form with post method, and this form is being posted using ajax request which call a function with if/else statement, like this:

    if(isset($_POST['save']))
    {
        $data['phase'] = 'translating';
    }
    elseif(isset($_POST['submit']))
    {
        $data['phase'] = 'waiting_approve';
    }

The if/else statement check which button is being clicked, and it works 100% after posting the data of the form in the usual way, but never works when posting it using ajax request.
My ajax request:

    $('#workspace-content').delegate('form#article', 'submit', function(){
    var that = $('form#article'),
        url = that.attr('action'),
        type = that.attr('method'),
        data = {};

        data = that.serialize();

        $.ajax({
            type: type,
            url : url,
            data : data,
            dataType: 'json',

            success: function(data){
                $('#header-search-field').append(data.msg).delay(3000).fadeOut(500, function(){
                    var that = $(this);
                    that.html('').fadeIn();
                });
            }
        });

    return false;
});

Any suggestion or solution?!!

The HTML form:

<button id="save-article" type="submit" name="save" class="btn btn-info btn-xs pull-left">
    <span class="glyphicon glyphicon-floppy-save"></span>
</button>
<input name="title" type="text" value="<?php echo $work->title;?>" class="form-control" id="title" placeholder="" />
<textarea row="10" name="article" class="form-control" id="article" placeholder=""><?php echo $work->article;?></textarea>
<button id="submit-article" type="submit" name="submit" class="btn btn-info btn-block">Send</button>
<input name="slug" type="hidden" value="<?php echo $work->slug;?>" />

When you submit a form normally, the button that you used to submit it will be included in the POST data. But when you submit with AJAX, and use that.serialize(), the POST data just includes the input fields -- the submit button isn't included. You need to attach your submit code to the buttons, so you can add the appropriate values to the data.

$('#workspace-content').on('click', 'form#article .btn', function(){
    var that = $(this).closest("form"),
        url = that.attr('action'),
        type = that.attr('method'),
        data = that.serialize();
    data += '&' + this.name + '=1';

    $.ajax({
        type: type,
        url : url,
        data : data,
        dataType: 'json',

        success: function(data){
            $('#header-search-field').append(data.msg).delay(3000).fadeOut(500, function(){
                var that = $(this);
                that.html('').fadeIn();
            });
        }
    });

    return false;
});