Xpath invalid expression that WORK!

Maybe try exchanging one set of single quotes for double?

e.g.

foreach ($sxml->xpath('/document/child[flag=".$flagValue."]/data') as $someData) {

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Greetings, Eslovo.  Here are some general guidelines to make your use of this site more enjoyable and productive for you...

At EE, the experts exchange answers and advice for points.  If you look at the questions awaiting answers in this zone, you will see a lot of 500 point questions.  Your question is competing for the experts' attention among those high-point questions.  So as a matter of simple economics you might be able to envision which questions will get the experts' attention first.   Just a thought.

We are experts, but not mind readers.  Inquiries that are vague may not get answers that are as succinct and effective as inquiries that have actual URLs, complete code examples, and clearly expressed questions.  Whenever possible, please provide the inputs and tell us what you want for the outputs.  An incredibly important concept is the SSCCE; please read the online page and embrace the SSCCE strategy.  If you do not have the SSCCE, stop what you're doing and create one, and post it with your question.  In the instant case, it looks like there is a mixup between the XML document and the PHP code.  Probably your setup doesn't really include that mixup, and we would be better able to help if you showed us the actual test data and the actual code.

Best regards, ~Ray

@kaufmed: Maybe try exchanging one set of single quotes for double?

$sxml->xpath("/document/child[flag=".$flagValue."]/data")

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Double quotas did not help.. Still get "Warning: SimpleXMLElement::xpath(): Invalid expression in"

Here is how I might do it: http://iconoun.com/demo/temp_eslovo.php

<?php // demo/temp_eslovo.php
ini_set('display_errors', TRUE);
error_reporting(E_ALL);

// SEE http://www.experts-exchange.com/Programming/Languages/Scripting/PHP/Q_28490788.html

// A SIMULATED XML DOCUMENT FROM THE POST AT EE
$xml = <<<EOD
<document>
    <child>
        <flag>1</flag>
        <data>First Item is to be Ignored</data>
    </child>
    <child>
        <flag>2</flag>
        <data>Second Item Data</data>
    </child>
</document>
EOD;

// THE SEARCH VALUE
$flagvalue = 2;

// MAKE AN OBJECT FROM THE XML DOCUMENT
$obj = SimpleXML_Load_String($xml);

// SEARCH THE OBJECT AND SHOW THE FINDINGS, IF ANY
foreach ($obj->child as $child)
{
    if ($child->flag != $flagvalue) continue;
    var_dump($child);
}

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HTH, ~Ray

@Ray Paseur It is not exactly what I was asking. Xpath is very powerful, but for some reason libxml on my laptop  with Fedora generates those annoying error messages. Xdebug just goes insane. Your solution simply puts xpath out of code.

I would like to keep xpath and be rid of errors. Though they do not appear in production environment. There're just no errors allowed to be desplayed. But web server log file is filled with this crap.

I wonder, may be I should remove php-topic and add linux instead? 'Cos the problem probably have something to do with installed libxml library...

libxml2-2.9.1-2.fc20.i686
libxml++-2.37.1-1.fc20.i686
libxml2-python-2.9.1-2.fc20.i686