This is the best method I've found for single asynchronous cURL's in PHP (great for calling external API's once your own has finished handling the request).

Asynchronous PHP cURL

function acURL($url, $fields = array(), $method = 'POST')
{
    # Set method
    $cmd = "curl -X ".$method;

    # Build fields
    foreach ($fields as $key => $value) {
        $cmd .= " -F '".$key."'='".$value."'";
    }

    # Finally append resource URL
    $cmd .= " '".$url."'";

    # Ouput stdout/err to nonexistent directory
    $cmd .= " > /dev/null 2>&1 &";

    # Execute
    exec($cmd, $output, $exit);

    // print_r($output); # Uncomment for debugging

    # Return
    return $exit == 0;
}