For loop while parsing xml in php?

I am trying to create a for loop that will parse out different elements of xml doc using php. It parses when I put the number of the entry in the line:

$s = $xml->entry[0]->children($namespaces['s']);

However I tried to replace the number with "i" to count through each of the entries in the xml doc and I get an error (Fatal error: Call to a member function children() on a non-object in /home/content/c/a/s/cashme/html/buylooper/xml.php on line 10). How do I resolve this?

<?php

$url = 'xml-file.xml'; 

$xml = simplexml_load_file($url); 

$namespaces = $xml->entry->getNameSpaces(true); 



for ($i = 0; $i <= 10; $i += 1){



$s = $xml->entry[i]->children($namespaces['s']); 

$title = $s->product->title; 



//print 

echo $retailer;



}

?>