You should always use api code, even if it's not required. Otherwise if you generate too much traffic, block chain will ban your ip for a day. You will not be able to use web wallet.

function convert_amount_to_btc($currency_from,$amount,$api_code) {

    $url = "https://blockchain.info/tobtc?api_code=".$api_code."&currency=".$currency_from."&value=".$amount;
    try 
    {
        $ch = curl_init();
        curl_setopt ($ch, CURLOPT_URL, $url);
        curl_setopt( $ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt ($ch, CURLOPT_USERAGENT, "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)");
        curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, 20);
        curl_setopt ($ch, CURLOPT_TIMEOUT, 20);
        $rawdata = curl_exec($ch);
        curl_close($ch);
        if(is_numeric($rawdata)){
            return array('status' => TRUE,'response' => $rawdata);
        }else{
            return array('status' => FALSE,'response' => $rawdata);
        }
    } 
    catch (Exception $e){
        return array('status' => FALSE,'response' => $e->Message());
    }
}

$app_api_key = "your-blockchain-api-key";//Your Own APIKey received from Blockchain

//BELOW AMOUNT IS CONVERTED FROM USD TO BTC 
$app_currency = "USD"; //App Local Currency - please always refer to the blockain docs for supported currencies

$usd_amount = "28";//USD
$convert = convert_amount_to_btc($app_currency,$usd_amount,$app_api_key);
//the response from convert can either be TRUE/FALSE | 
// $convert['status'] (True means the amount was converted, False meaning there was an error/issue 'reason like not a supported currency or your call server IP being blocked') 
// $convert['response'] - carries amount convert - on error carries blockchain call reponse message
if((bool)$convert['status']){
    $amount = $convert['response'];//Amount converted from fiat currency to BTC
}else{
    //here for error purposes it up to your App what to do
    //your can display a message or ask the user to try again
}