There may be cases where a null $result is legitimate, in this case add a test for non-zero xlst_errno:
<?php
$xh = xslt_create();
$result = xslt_process($xh, 'dog.xml', 'pets.xsl');
if (!$result && xslt_errno($xh) > 0) {
die(sprintf("Cannot process XSLT document [%d]: %s",
xslt_errno($xh), xslt_error($xh)));
}
echo $result;
xslt_free($xh);
?>
xslt_errno