Php SimpleXML和foreach

我在PHP和XML方面遇到了一些问题。如果需要使用

FOREACH

从XML源中筛选特定参数，并且特定块不存在，则结果为空。但我需要展示这样的东西：“没有现役”

to作为$to）{
如果（$to->form=='pswA'&&&$to->heading=='A'）{echo“allow”}
elseif（$to->form='pswA'&&&$to->heading='B'）{echo“notallow”}
elseif（$to->form='pswA'&&&$to->heading='C'）{echo“block”}
}//此存在并显示（允许/notallow/block）
foreach（$xml->to as$to）{
如果（$to->form=='pswC'&&&$to->heading=='A'）{echo“allow”}
elseif（$to->form=='pswC'&&&$to->heading=='B'）{echo“notallow”}
elseif（$to->form='pswC'&&&$to->heading='C'）{echo“block”}
}//但这是空的。。。我怎样才能显示一些回声？
?>

---

当第二个

foreach

不存在时，是否可以显示消息？此示例和live preview中的更多信息：

使用变量跟踪是否找到了活动服务

<?php
$data = '<?xml version="1.0" encoding="utf-8" ?>
<note>
  <to>
    <form>pswA</form>
    <heading>A</heading>
  </to>
  <to>
    <form>pswB</form>
    <heading>A</heading>
  </to>
</note>';

$xml = simplexml_load_string($data);
  foreach ($xml->to as $to){
      if     ($to->form == 'pswA' && $to->heading == 'A'){echo "allow";}
      elseif ($to->form == 'pswA' && $to->heading == 'B'){echo "notallow";}
      elseif ($to->form == 'pswA' && $to->heading == 'C'){echo "block";}
  } // this exist and show (allow / notallow / block)

  foreach ($xml->to as $to){
      if     ($to->form == 'pswC' && $to->heading == 'A'){echo "allow";}
      elseif ($to->form == 'pswC' && $to->heading == 'B'){echo "notallow";}
      elseif ($to->form == 'pswC' && $to->heading == 'C'){echo "block";}
  } // but this is empty... how can I show some echo?
?>

$xml = simplexml_load_string($data);
$pswA_found = false;
foreach ($xml->to as $to){
    if     ($to->form == 'pswA' && $to->heading == 'A'){
        echo "allow";
        $pswA_found = true;
    }
    elseif ($to->form == 'pswA' && $to->heading == 'B'){
        echo "notallow";
        $pswA_found = true;
    }
    elseif ($to->form == 'pswA' && $to->heading == 'C'){
        echo "block";
        $pswA_found = true;
    }
}
if (!$pswA_found) {
    echo "No active service for pswA";
}

$pswC_found = false;
foreach ($xml->to as $to){
    if     ($to->form == 'pswC' && $to->heading == 'A'){
        echo "allow";
        $pswC_found = true;
    }
    elseif ($to->form == 'pswC' && $to->heading == 'B'){
        echo "notallow";
        $pswC_found = true;
    }
    elseif ($to->form == 'pswC' && $to->heading == 'C'){
        echo "block";
        $pswC_found = true;
    }
}
if (!$pswC_found) {
    echo "No active service for pswC";
}