Improving Geocoding Accuracy using MapQuest Open Data

I have written the following function in php to geocode a customers' address from the MapQuest API. I know that google is far better, but I need to use this for a commercial project and want to use a free solution.

I am getting results for about 40-50% of addresses I try to geocode. (I have tried about 20 so far). (I haven't verified the results of the accuracy of the addresses, just the fact that I got a result).

Is there any way I can improve the geocoding results using their API. I don't want to share addresses as that is private information, but for all the addresses I supply all the parameters except country and address_2.

function geocode_customer($customer_id)
{
    $customer_info = $this->get_info($customer_id);
    $mapquest_api_key = 'MY%API%KEY';       
    $geocode_base_api_url = 'http://open.mapquestapi.com/geocoding/v1/address';

    $params = array();

    $params['key'] = $mapquest_api_key;
    $params['outFormat'] = 'json';
    if ($customer_info->country)
    {
        $params['country'] = $customer_info->country;
    }

    if ($customer_info->state)
    {
        $params['state'] = $customer_info->state;
    }

    if ($customer_info->zip)
    {
        $params['postalCode'] = $customer_info->zip;
    }

    if ($customer_info->city)
    {
        $params['city'] = $customer_info->city;
    }

    if ($customer_info->address_1)
    {
        $street = $customer_info->address_1;

        if ($customer_info->address_2)
        {
            $street.= ' '.$customer_info->address_2;
        }
        $params['street'] = $street ;
    }

    $params['format'] = 'json';

    $params_string = '';
    //url-ify the data for the POST
    foreach($params as $key=>$value) 
    { 
        $value = $key == 'key' ? $value : rawurlencode($value);
        $params_string .= $key.'='.$value.'&'; 
    }
    $params_string = rtrim($params_string, '&');


    $api_url = $geocode_base_api_url.'?'.$params_string;

    $c = curl_init();
    curl_setopt($c, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($c, CURLOPT_URL, $api_url);
    $contents = curl_exec($c);
    curl_close($c);

    $resp = json_decode($contents, true);

    if(!empty($resp['results'][0]['locations']))
    {
        $latLng = $resp['results'][0]['locations'][0]['latLng'];
        return $latLng['lat'].','.$latLng['lng'];
    }

    //Default to Denver CO
    return '39.737567,-104.9847179';
}

Copyright Notice：Content Author:「Chris Muench」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/21868673/improving-geocoding-accuracy-using-mapquest-open-data