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).'
'; //会4舍5入
echo number_format($num,2).'
';
echo number_format($num,2,':').'
'; //这种写法是错误的
echo number_format($num,2,':','-').'
';
?>
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