code: function useSingleQuote(node, options) {
    return (
      !node.isDoubleQuote ||
      (options.singleQuote &&
        !node.raw.match(/\\n|\\t|\\r|\\t|\\v|\\e|\\f/) &&
        !node.value.match(
          /["'$\n]|\\[0-7]{1,3}|\\x[0-9A-Fa-f]{1,2}|\\u{[0-9A-Fa-f]+}/
        ))
    );
  }
docs: Check if string can safely be converted from double to single quotes, i.e.
  
  - no embedded variables ("foo $bar")
  - no linebreaks
  - no special characters like \n, \t, ...
  - no octal/hex/unicode characters
  
  See http://php.net/manual/en/language.types.string.php#language.types.string.syntax.double