Twitter bootstrap collapse: change display of toggle button

Question

I am using Twitter Bootstrap to create collapsible sections of text. The sections are expanded when a + button is pressed. My html code as follows:

<div class="row-fluid summary">
    <div class="span11">
        <h2>MyHeading</h2>  
    </div>
    <div class="span1">
        <button type="button" class="btn btn-success" data-toggle="collapse" data-target="#intro">+</button>
    </div>
</div>
<div class="row-fluid summary">
    <div id="intro" class="collapse"> 
        Here comes the text...
    </div>
</div>

Is there a way to change the button to display - instead of + after the section is expanded (and change back to + when it is collapsed again)?

Additional information: I hoped there would be a simple twitter-bootstrap/css/html-based solution to my problem. All responses so far make use of JavaScript or PHP. Because of this I want to add some more information about my development environment: I want to use this solution inside a SilverStripe-based (version 3.0.5) website which has some implications for the use of both PHP as well as JavaScript.

Answer 1

try this. http://jsfiddle.net/fVpkm/

Html:-

<div class="row-fluid summary">
    <div class="span11">
        <h2>MyHeading</h2>  
    </div>
    <div class="span1">
        <button class="btn btn-success" data-toggle="collapse" data-target="#intro">+</button>
    </div>
</div>
<div class="row-fluid summary">
    <div id="intro" class="collapse"> 
        Here comes the text...
    </div>
</div>

JS:-

$('button').click(function(){ //you can give id or class name here for $('button')
    $(this).text(function(i,old){
        return old=='+' ?  '-' : '+';
    });
});

Update With pure Css, pseudo elements

http://jsfiddle.net/r4Bdz/

Supported Browsers

button.btn.collapsed:before
{
    content:'+' ;
    display:block;
    width:15px;
}
button.btn:before
{
    content:'-' ;
    display:block;
    width:15px;
}

Update 2 With pure Javascript

http://jsfiddle.net/WteTy/

function handleClick()
{
    this.value = (this.value == '+' ? '-' : '+');
}
document.getElementById('collapsible').onclick=handleClick;