Instead of creating a new empty div, add the div to the document, e.g.:
<body>
<div id="dialog">
<div id="dialogContent"></div>
</div>
</body>
Then, your script will become
var dialog_list = $("#dialog").dialog({ });
Then, when you want to change the HTML of that element, instead of changing the dialog itself, you'd want to change the dialogContent element:
function showtimes_list(sid){
var content = $("#dialogContent");
content.html("");
$.get("ajax_showtimes.php?sid="+sid, function(data){
content.html(data);
}
);
dialog_list.dialog("open");
}
If you don't want these empty divs in your HTML structure, you should add them to the body dynamically and use classes instead of ids.
Edit: to answer your question as to why it doesn't work when the dialog_list is outside the function, I'd imagine it has something to do with the generated html.
When you call .dialog(), jQuery generates the following HTML:
<div class="ui-dialog ui-widget ui-widget-content ui-corner-all ui-draggable ui-resizable">
<div class="ui-dialog-titlebar ui-widget-header ui-corner-all ui-helper-clearfix">
<span id="ui-dialog-title-dialog" class="ui-dialog-title">Dialog title</span>
<a class="ui-dialog-titlebar-close ui-corner-all" href="#"><span class="ui-icon ui-icon-closethick">close</span></a>
</div>
<div style="height: 200px; min-height: 109px; width: auto;" class="ui-dialog-content ui-widget-content" id="dialog">
<p>Dialog content goes here.</p>
</div>
</div>
When this is outside of your function, it is called whenever it is encountered in the script... Then, in your function, you change that generated HTML. It's been a while since I dynamically updated dialog content, but I ran into the same problem, and the generated HTML was the culprit. I think my solution was to, instead of the nested divs in my original answer, I created the dialog as you did (outside the function), and inside the function, you change the html of the ui-dialog-content
For instance:
function showtimes_list(sid){
dialog_list.find('.ui-dialog-content').html("");
$.get("ajax_showtimes.php?sid="+sid, function(data){
dialog_list.find('.ui-dialog-content').html(data);
}
);
dialog_list.dialog("open");
}