You don't really need xpath here. You can process this XML using the SimpleXMLElement php class like this:
<pre>
<?php
$xmlStr = '<offers>
<offer><id>1578</id>
<instock>13</instock>
</offer>
<offer><id>1579</id>
<property name="EAN">634932593250256</property>
<property name="Dostępne kolory">Niebiesko-Biały</property>
<property name="Dostępne rozmiary">L</property>
<instock>1</instock>
</offer>
</offers>';
$xml = new SimpleXMLElement($xmlStr);
foreach ($xml->offer as $offer)
{
print_r($offer);
}
Outputs:
SimpleXMLElement Object
(
[id] => 1578
[instock] => 13
)
SimpleXMLElement Object
(
[id] => 1579
[property] => Array
(
[0] => 634932593250256
[1] => Niebiesko-Biały
[2] => L
)
[instock] => 1
)