问php SimpleXMLElement奇怪的行为

EN

    $sql = array();
    foreach($this->current_file as $c => $record){
        $sql[$c] = array();

        foreach($this->schema['tables'][$this->current_file_name]['pairing'] as $sql_field => $xml_field){
            $sql[$c][$sql_field] = $record->$xml_field;
        }
    }

XML field (echo $xml_field):                  external_id
SQL field (echo $sql_field):                  global_user_id
External ID (echo $record->external_id):      0717d6d0-3ffb-427d-8870-890132191e7b
Dynamic XML field (echo $record->$xml_field): 

Dump of $record:
    object(SimpleXMLElement)[29]
      public 'external_id' => string '0717d6d0-3ffb-427d-8870-890132191e7b' (length=36)
      public 'teacher' => string 'T' (length=1)
      public 'username' => string 'jdoe' (length=9)
      public 'ChosenName' => string 'John' (length=8)
      public 'Surname' => string 'Doe' (length=8)
      public 'Title' => string 'mr' (length=2)
      public 'Gender' => string 'male' (length=4)

Dump of $record->$xml_field ($record->external_id):
    object(SimpleXMLElement)[30]

echo $record->external_id;

$xml_field = 'external_id';
echo $record->$xml_field;

$xml_element = 'external_id';
var_dump($record->$xml_element);

// Outputs:
[external_id] => SimpleXMLElement Object
    (
        [0] => 0717d6d0-3ffb-427d-8870-890132191e7b
    )