$doc = new DOMDocument('1.0', 'iso-8859-1');
$doc->load($xmlPath);

$myXMLString = file_get_contents($xmlPath);
$myXMLString = utf8_decode($myXMLString);
$doc = new DOMDocument('1.0', 'iso-8859-1');
$doc->loadXML($myXMLString);

<?php
// german Umlaut ä in latin1 = 0xE4
$xml = '<foo>'.chr(0xE4).'</foo>';

$doc = new DOMDocument;
$b = $doc->loadxml($xml);
echo 'with doc->recover=false(default) : ', ($b) ? 'success':'failed', "n";

$doc = new DOMDocument;
$doc->recover = true;
$b = $doc->loadxml($xml);
echo 'with doc->recover=true : ', ($b) ? 'success':'failed', "n";

Warning: DOMDocument::loadXML(): Input is not proper UTF-8, indicate encoding !
Bytes: 0xE4 0x3C 0x2F 0x66 in Entity, line: 1 in test.php on line 6
with doc->recover=false(default) : failed

Warning: DOMDocument::loadXML(): Input is not proper UTF-8, indicate encoding !
Bytes: 0xE4 0x3C 0x2F 0x66 in Entity, line: 1 in  test.php on line 11
with doc->recover=true : success

<?php
$xml = sprintf('<foo>
    <ae>%s</ae>
    <oe>%s</oe>
    &
</foo>', chr(0xE4),chr(0xF6));

libxml_use_internal_errors(true);
$doc = new DOMDocument;
$doc->recover = true;
libxml_clear_errors();
$b = $doc->loadxml($xml);
$invalidCharFound = false;
foreach(libxml_get_errors() as $error) {
    if ( 9==$error->code && !$invalidCharFound ) {
    	$invalidCharFound = true;
    	echo "found invalid char, possibly harmlessn";
    }
    else {
    	echo "hm, that's probably more severe: ", $error->message, "n";
    }
}