Magento – Magento SOAP 500 Internal Error Response

I'm getting a 500 internal error response when I try to retrieve data via SOAP.

What I have that's working and not giving me 500 internal error:

<?php
$client = new SoapClient('http://example.com/api/v2_soap/?wsdl');
$session = $client->login('xxx', 'xxx');
$complexFilter = array(
    'complex_filter' => array(
        array(
            'key' => 'type',
            'value' => array('key' => 'in', 'value' => 'simple,configurable')
        )
    )
);
$result = $client->catalogProductList($session, $complexFilter);

foreach ($result as $key => $value) {
    $id = $value->product_id;
    echo $id;
   // productPrices($id); <-- this issue causes the error.
}



?>

When I then try to call a funtion in the foreach loop (see comment above):

function productPrices($id){
    $client = new SoapClient('http://example.com/api/soap/?wsdl');
    $session = $client->login('xxx', 'xxx');
    $att = array("visibility","sku","special_price", "price");
    $arguments = array( $id, NULL, $att);
    $resultPrice = $client->call($session, 'catalog_product.info', $arguments);
    echo $resultPrice['visibility'].",".$resultPrice['sku'].",".$resultPrice['special_price'].",".$resultPrice['price'];
}

The response I get from the server is now internal error 500. I have maximum 90 sku's/product ids.

Anyone know what the error might be?