A ver:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento sin título</title>
<script type="text/javascript">
function a(){alert('se ejecuta la función a');}
function b(){alert('se ejecuta la función b');}
</script>
</head>
<body>
<input name="pp" type="checkbox" value="" id="pp" /><br />
<a href="#" onclick="if(document.getElementById('pp').checked){a();}else{b();}return false;">test</a>
</body>
</html>