snippet string-parameter is not compared

[[!getCardFields? &field='content']]

$string = $field;
if ($string == "content"){
    echo 'good';      // doesn't enter into if, why???
}
 
$string = "content";
if ($string == "content"){
    echo 'good';      // enters into if, ok
}
 
echo $field;          // "content"

$string = $modx->getOption('field', $scriptProperties, 'default-field');