<?php
$xml = '<cars> 
            <brand name="Audi"> 
                <model>A1</model> 
                <model>A3</model>
                <model>A5</model> 
            </brand> 
            <brand name="Ferrari"> 
                <model>F12</model>
                <model>FF</model> 
            </brand> 
        </cars>';

$doc = simplexml_load_string($xml);

foreach ($doc->children() as $brand) {
    foreach ($brand->children() as $model) {
        $cars[(string)$brand->attributes()->name][] = (string)$model;
    }
}

echo '<pre>';
print_r($cars);
echo '</pre>';
?>