递归函数，求高手解原理。解决方法

递归函数，求高手解原理。
function demo($num)
{
echo $num."
";
if($num > 0 ){
demo($num -1 );
}else{
echo "--------------
";
}
echo $num."
";
}

demo(10);

------解决方案--------------------
http://blog.csdn.net/zhangchao3322218/article/details/6835000
http://www.blogjava.net/flysky19/articles/95785.html