php 如何把txt文件的内容放到数组$arr里面

a.txt内容如下：
3 12 1 3 8 10 2 3 11 2 8 10
只有一行，这里只写部分数据，没有换行等符号，只是在数据中间有空格分开，用php怎么读取，以空格为分割，把这些值写到数组$arr里面，即为如下的效果
$arr = array ("3","12","1","3","8","10","2","3","11","2","8","10")

$arr = explode(' ',file_get_contents('a.txt'));