Check for existing values with if in foreach?

I have array with already existing values:

$existingValues = array();

now i get new values in xml file (is a import script) but i have to avoid insert of already existing values, my question is, how can i do a if check in foreach where i list all new values from xml?

$i = 1;

foreach($node->children() as $child) :
    $attribute2 = $child->attributes();
    $productcode = $attribute2['sku'];
    $productvariant = $attribute2['variantid'];
    $productprice = $attribute2['price'];

    if ($attribute2['sku']) :
        echo $productcode . ' - ' . $productvariant . '<br>';
    endif;

    $i++;      
endforeach;

I tried with in_array() but it's not correct.