For the above recursion, why is the last $ A value 0? Not 1?

For the following recursion, why is the last $ A value 0? Isn't it 1?

PHP Code

  
   
  
   ';    $a + +;    if ($a < 2) {         Test ();    }    $a--;    return $a;}? >

For the above recursion, why is the last $ A value 0? Isn't it 1?
I debugged with debugger and found that the order of operation was:
Order: 4-5-6-7-8-4-5-6-7-10-11-10-11
$a value: 0-0-1-1-1-1-1-2-2-1-1-0-0
The result of the last run output is
0
1
0
Just want to ask, why do 10 rows and 11 rows run two times?

Thanks, man.

------Solution--------------------
This execution is sequential
How many times do you say that?
4 static $a = 0;//This is line 4th.
5 echo $a. '
';
6 $a + +;
7 if ($a < 2) {
8 Test ();
4 static $a = 0;//This is line 4th.
5 echo $a. '
';
6 $a + +;
7 if ($a < 2) {
9}
Ten $a--;
return $a;
9}
Ten $a--;
return $a;

------Solution--------------------

PHP code

function Test () {static $a = 0;//here is line 4th------------ ------------(1) echo $a.    ' 
 ';    $a ++;/* Note that the next line in the non-commented code, the return value must be defined at (1) *//* If you want to get the expected return result 1, if ($a < 2) {return Test ();    }*/$a--; return $a;} 
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