它将"3美元"转换为数字，得到$a = 3.

It casts '3dollars' as a number, getting $a = 3.

在回显时，在$a上加上20，因此将打印23和$a = 23.

When you echo, you add 20, to $a, so it prints 23 and $a = 23.

然后，当您打印时，再次添加20，所以现在是$a = 43.

Then, when you print, you again add 20, so now $a = 43.