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Date: Fri, 27 Feb 2004 13:37:15 -0500 (EST)
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Subject: SimpleXML & Casting to String
From: adam@trachtenberg.com (Adam Maccabee Trachtenberg)

I know we discussed this already, but after seeing a couple of bug
reports about SimpleXML, I'm worried our decision only makes sense to
us and not to regular users. :)

Specifically, since elements and attibutes look like strings, people
expect them to act like strings. But since they're not objects instead
of strings, they're completely buffled as how to handle them.

Here are two examples that have come through the bug report system in
the last day:

$sxe = simplexml_load_string('<root a="123" />');
if ($sxe['a'] == '123') {
  // do something
}

And:

$xml = simplexml_load_string(/* some valid XML string that I'm not
                                going to cut and paste here */);
foreach($xml->user as $user){
  if (utf8_decode($user->login) == $login &&
      utf8_decode($user->password) == $password) {
      // valid users
  }
}

Both seem like they should work, but neither do.

In the first example, we're comparing an object with a string. Even
though $sxe['a']->__toString() == 'a', the comparison fails. (Well, you
can't actually do that, but you know what I mean.)

In the second example, utf8_decode() expects a string and not an
object and we again we don't autoconvert.

The problems can be solved by explicitly casting the object to a
string, but since you rarely need to cast elsewhere in PHP, I don't
think anyone thinks of it as a necessary step.

Originally, I proposed that PHP autoconvert an object to a string
whenever the object has a __toString() and it's necessary to treat the
variable as a string. In the first example, since we're comparing an
object to a string, we would cast down the object to enable the
comparison. This would work just like 1 == '1'. In the second case,
since the function expects a string, we'd also do the cast.

However, Andi (and others) raised some valid issues about edge cases
and other potential engine problems.

Is there anything we can do to help out people so that SimpleXML works
as they expect, but doesn't have the potential to unleash hell on PHP
and the bug system?

Maybe it makes sense to have SimpleXML leaf nodes return as strings
instead of SimpleXML objects? Or does this merely substitute one set
of problems for another? (E.g. this breaks iteratation, what happens
when there's multiple leaves, etc.)

I don't know what the right answer is, but I feel that the current
solution isn't perfect. It may end up to be the best possible method,
but I'm not yet convinced it is.

-adam

-- 
adam@trachtenberg.com
author of o'reilly's php cookbook
avoid the holiday rush, buy your copy today!