Description:
------------
If you are using php functions inside an XSL script and return 'false', then XSL sees it as 'true', and vice versa.

Test script:
---------------
PHP script

$xslDoc = new DOMDocument();
$xslDoc->load('Stubs/php-functions.xsl');

$xmlDoc = new DOMDocument();
$xmlDoc->load('Stubs/collection.xml');

$native = new \XSLTProcessor();
$native->importStylesheet($xslDoc);
$native->registerPHPFunctions();
$nativeResult = trim($native->transformToXML($xmlDoc));

$transpiler = new XsltProcessor();
$transpiler->importStylesheet($xslDoc);
$transpiler->registerPHPFunctions();
echo $transpiler->transformToXML($xmlDoc);

XML Document

<collection />

XSL Sheet

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:php="http://php.net/xsl">
    
    <xsl:output method="xml"
                indent="no"
                omit-xml-declaration="yes"
                encoding="UTF-8"/>

    <xsl:template match="collection">
        <xsl:value-of select="php:functionString('strpos', 'Hello', 'World')" />
    </xsl:template>

</xsl:stylesheet>

Expected output: false
Actual output: true

Expected result:
----------------
false

Actual result:
--------------
true