Limits to XPath implementaiton in PHP5

Poking around with XPath using SimpleXML, it looks like there are at
least a few reasonably common XPath operators and predicates that
aren't supported. I'd like to check my observations against other
people's experience, and find out if I'm missing something.

Here's what it looks like to me in PHP 5.2.0:

* the count() predicate seems to fail quietly (no error or warning,
you just receive a false value from the xpath method rather than an
array)
* the "|" (union) operator seems to also fail quietly by returning an
empty array
* the "union" operator seems to cause an error
* the "intersect" operator also causes an error (and needless to say,
with | and count() not working, using something like the Kaysian
technique to get the intersection won't work)

Does this accurately capture the current state of things?

If so, are the bulk of developers simply not using these features, or
are they instead writing around the missing features in PHP rather
than trying to get XPath to do the work?

Or is there a better option?

Re: Limits to XPath implementaiton in PHP5

Weston wrote:
> Poking around with XPath using SimpleXML, it looks like there are at
> least a few reasonably common XPath operators and predicates that
> aren't supported. I'd like to check my observations against other
> people's experience, and find out if I'm missing something.
>
> Here's what it looks like to me in PHP 5.2.0:
>
> * the count() predicate seems to fail quietly (no error or warning,
> you just receive a false value from the xpath method rather than an
> array)
> * the "|" (union) operator seems to also fail quietly by returning an
> empty array
> * the "union" operator seems to cause an error
> * the "intersect" operator also causes an error (and needless to say,
> with | and count() not working, using something like the Kaysian
> technique to get the intersection won't work)
>
> Does this accurately capture the current state of things?
>
> If so, are the bulk of developers simply not using these features, or
> are they instead writing around the missing features in PHP rather
> than trying to get XPath to do the work?
>
> Or is there a better option?
>

AFAIK, the various PHP API's (SimpleXML, DOM) use libxml as a backend,
without much logic in between. So if the XPath operators are supported
by libxml, they ought to be supported by PHP. If they're not supported
by libxml, that would really surprise me.

That said, try using the DOM API instead of SimpleXML and see if you
have better results. Then, try peeking into the libxml error stack
directly using the libxml functions:

http://us.php.net/manual/en/ref.libxml.php

Jeremy

Re: Limits to XPath implementaiton in PHP5

Weston wrote:
> Poking around with XPath using SimpleXML, it looks like there are at
> least a few reasonably common XPath operators and predicates that
> aren't supported. I'd like to check my observations against other
> people's experience, and find out if I'm missing something.
>
> Here's what it looks like to me in PHP 5.2.0:
>
> * the count() predicate seems to fail quietly (no error or warning,
> you just receive a false value from the xpath method rather than an
> array)
> * the "|" (union) operator seems to also fail quietly by returning an
> empty array
> * the "union" operator seems to cause an error
> * the "intersect" operator also causes an error (and needless to say,
> with | and count() not working, using something like the Kaysian
> technique to get the intersection won't work)
>
> Does this accurately capture the current state of things?
>
> If so, are the bulk of developers simply not using these features, or
> are they instead writing around the missing features in PHP rather
> than trying to get XPath to do the work?
>
> Or is there a better option?
>

AFAIK, the various PHP API's (SimpleXML, DOM) use libxml as a backend,
without much logic in between. So if the XPath operators are supported
by libxml, they ought to be supported by PHP. If they're not supported
by libxml, that would really surprise me.

That said, try using the DOM API instead of SimpleXML and see if you
have better results. Then, try peeking into the libxml error stack
directly using the libxml functions:

http://us.php.net/manual/en/ref.libxml.php

Jeremy

Re: Limits to XPath implementaiton in PHP5

On Oct 22, 3:37 pm, Jeremy wrote:
> That said, try using the DOM API instead of SimpleXML and see if you
> have better results.

There are indeed some different results. Here's a basic rundown:

count() predicate: succesful under DOM, fails quietly under SimpleXML
"|" (union) operator: succesful under DOM, not sure under SimpleXML
"union" operator: fails with error under both
"intersect" operator: fails with error under both
kaysian intersection technique: succesful under DOM, fails under
SimpleXML

>Then, try peeking into the libxml error stack
> directly using the libxml functions:
>
> http://us.php.net/manual/en/ref.libxml.php

I'll have to poke at that further, but at this point it does look like
both APIs can't simply be handing off to libxml.

Re: Limits to XPath implementaiton in PHP5

On Oct 22, 3:37 pm, Jeremy wrote:
> That said, try using the DOM API instead of SimpleXML and see if you
> have better results.

There are indeed some different results. Here's a basic rundown:

count() predicate: succesful under DOM, fails quietly under SimpleXML
"|" (union) operator: succesful under DOM, not sure under SimpleXML
"union" operator: fails with error under both
"intersect" operator: fails with error under both
kaysian intersection technique: succesful under DOM, fails under
SimpleXML

>Then, try peeking into the libxml error stack
> directly using the libxml functions:
>
> http://us.php.net/manual/en/ref.libxml.php

I'll have to poke at that further, but at this point it does look like
both APIs can't simply be handing off to libxml.