About _ set

Class & nbsp; aa {function & nbsp ;__ set ($ pro, & nbsp ;v v) {echo & nbsp;; $ this-& gt; pro $ v; echo & nbsp; $ vn ;}$ anew & nbsp; aa; $ a-& gt; b1; output is a & nbsp; a & nbsp; 1 & nbsp; 1; 2 questions, why about _ set

Class aa {
Function _ set ($ pro, $ v ){
Echo "a \ n ";
$ This-> pro = $ v;
Echo "$ v \ n ";
}
}

$ A = new aa;
$ A-> B = 1;

The output is a 1 1;

Why are two problems output;

Why is it a 1 1, not a 1 a 1;


------ Solution ----------------------
Change echo "a \ n" to echo "$ pro \ n ".
B enters the _ set method because the B attribute does not exist.
Pro also enters the _ set method again because the pro attribute does not exist.
It is not surprising to print the value twice after entering 1.
1

When $ this-> pro = $ v;
Change to $ this-> $ pro = $ v;
Only once