相关疑难解决方法(0)

$string1 = "This is test [example]";
$string2 = "This is test [example][2]";
$string3 = "This [is] test [example][3]";

我怎样才能得到以下结果？

For $string1 -> example
For $string2 -> example*2
For $string3 -> is*example*3