为什么在第三次点击后我的功能不起作用？

I have the following html code:

<?php foreach ($this->tags as $uri=>$tag){?>
    <input type="checkbox" name="tags[]" style="display: none;" value="<?php echo $uri;?>" id="create_<?php echo $uri;?>" <?php echo isset($args['tags']) && in_array($uri, $args['tags'])?'checked="checked"':'';?> />
    <span onclick="selectTag(this.id)" id="create_<?php echo $uri;?>" for="create_<?php echo $uri;?>" class="tag <?php echo isset($args['tags']) && in_array($uri, $args['tags'])?'selected':'';?>"><?php echo str_replace(' ', '&nbsp;', $tag);?></span>
<?php }?>

And here is my JS code:

function selectTag(id) {
    var input = '.tags input#'+id;
    var span = '.tags span#'+id;
    if ($(input).is(':checked') && $(span).hasClass('selected')) {
        $(span).removeClass('selected');
        $(input).attr('checked', false);
    }
    else {
        $(span).addClass('selected');
        $(input).attr('checked', true);
    }
}

When I click on a span box, selects the box, and when I click again on it, it unselects it. The problem is, that after the 3rd time, it just stops working.

What is wrong with my code that is not working?

JQuery now has a prop method that is a slightly better alternative to using the attr method.

Try replacing your calls with attr("checked", true); with calls to prop("checked", true);

See here for documentation on prop: http://api.jquery.com/prop/

Here for a discussion between the two: .prop() vs .attr()

Edit:

Also, as Ed Cottrell stated, you'll want to have UNIQUE id attributes for all your elements on your page.

Edit2:

I have created a fiddle that demonstrates this usage: http://jsfiddle.net/xDaevax/E39hc/

You are giving the input and the span the same id attribute. Ids must be unique per element; you cannot have an input and a span the same id. Doing it this way will cause all sorts of problems, including the behavior you are experiencing. Give one of them a slightly different id (like create_<?php echo $uri;?>_span).

Also, as @xDaevax says, you should use .prop rather than .attr -- I have had the same problem when using .attr.