我可以将12345678.9输出成12,345,678.90吗?

当然可以,PHP中有个函数number_format,函数原型:
        string number_format
        (
           float number,
           int decimals,
           string dec_point,
           string thousands_sep
        )
    举个例子:
        <?
        $num = 12345678.9;
        echo number_format($num).'<br>'; //会4舍5入
        echo number_format($num,2).'<br>';
        echo number_format($num,2,':').'<br>'; //这种写法是错误的
        echo number_format($num,2,':','-').'<br>';
        ?>
    输出结果为:
        12,345,679
        12,345,678,90
        Warning: Wrong parameter count for number_format() in /data/homepage/test/number_format.php on line 5
        12-345-678:90

补充：Php教程,常见问题