Description:
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Using the DOMText function splitText() on a text node containing multibyte characters results in the node being split at the wrong position.

Reproduce code:
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$text = 'This is an ?example? of using DOM splitText';
$start = 30;
$length = 3;

$dom = new DOMDocument('1.0', 'UTF-8');
$node = $dom->createTextNode($text);
$dom->appendChild($node);

print "Text: $node->textContent\n";

print 'Expected (mb_substr): ' . mb_substr($text, $start, $length, 'UTF-8') . "\n";

$matched = $node->splitText($start);
$matched->splitText($length);
print "Actual (splitText): $matched->textContent\n";

Expected result:
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Text: This is an ?example? of using DOM splitText
Expected (mb_substr): DOM
Actual (splitText): DOM


Actual result:
--------------
Text: This is an ?example? of using DOM splitText
Expected (mb_substr): DOM
Actual (splitText): ing