Source
File: wp-includes/class-http.php
public function _get_first_available_transport( $args, $url = null ) {
$transports = array( 'curl', 'streams' );
$request_order = apply_filters( 'http_api_transports', $transports, $args, $url );
foreach ( $request_order as $transport ) {
if ( in_array( $transport, $transports, true ) ) {
$transport = ucfirst( $transport );
}
$class = 'WP_Http_' . $transport;
if ( ! call_user_func( array( $class, 'test' ), $args, $url ) ) {
continue;
}
return $class;
}
return false;
}