In php I get a different scope for a variable inside a condition - how to fix it?

Question

WordPress

Hello!
The online store has fields that receive values ​​from the $_POST form , including billing_email1, billing_email2, billing_email3 , etc. depending on the given amount. (there are other fields too, but we are not interested here)
My task is to write the incoming values ​​into the fields:
In billing_email , the first value is billing_email1 , and in billing_email_dop , all subsequent billing_email2, billing_email3 , etc. through a semicolon.
I've been fighting for a day now, I just can't figure out what is the reason why $key outside and inside the conditions turn out to be different ..

<?php	
    $fields = $checkout->get_checkout_fields( 'billing' );

  foreach ( $fields as $key => $field ) {
    $i = 1;
    $postdata = "";
    $postdop = "";
    $postemail = "";
    var_dump($key); // $key = billing_email_dop есть
    if($_POST["$key"."$i"]) {
      var_dump($key); // $key = billing_email_dop уже нет, вместо него $key = billing_email - почему?
      while($_POST["$key"."$i"]) { 
        
        if($key==="billing_email") {
          if(("$key"."$i")=="billing_email1") {
            $postemail = $_POST['billing_email1'];
          }
          else {
            $postdop .= $_POST["$key"."$i"]."; "; //в $postdop значение записывается корректно
          }
        }
        else {
          $postdata .= $_POST["$key"."$i"]."; ";
        }
        $i++;
      }
    }
      if($key=="billing_email") {
        woocommerce_form_field( $key, $field, $postemail );
      }
      else if($key=="billing_emaildop") {
        woocommerce_form_field( $key, $field, $postdop );
      }
      else {
        woocommerce_form_field( $key, $field, $postdata );
      }
  } 
?>

Answer 1

wardump $fields after $checkout->get_checkout_fields( 'billing' ); otherwise it’s not clear what you get there, maybe it’s a resource in general ...
PS: add if($_POST["$key"."$i"]) {...} else{var_dump($key);} and everything will fall into place.
PPS: in general, the code, of course, besides being built feet first, is replete with both obvious errors and implicit (for beginners) glitches ... for example , billing_email_dop eventually becomes billing_emaildop , I am silent about skipping iterations inside conditional operators, and as Maxim Timofeev pointed out - cyclically overwritten data
In short, a classic example of how NOT to do it.
How to change:
1) It makes no sense to build a loop on $fields, you clearly know its structure, unlike the data from $_POST.
2) In the $_POST loop, look for keys that match the pattern (for example, checking for the billing_email substring or the billing_email\d{1,2} regular expression). If exactly billing_email1 - write to $postemail, otherwise $postdop[] = $val; On the client, the form fields should be named more clearly, for example, billing_email[], after which you can immediately get the ENTIRE array of emails.
3) at the end if $postdop is not empty - multiply $postdop values ​​with commas, get your string with emails separated by commas.
4) assign everything to the required fields of your $fields.
5) Profit.

Answer 2

What does woocommerce_form_field(...) look like?
Apparently, there the first parameter is passed by reference, and it changes there. Because it is different in the cycle.