Source

File: wp-includes/class-http.php

public function _get_first_available_transport( $args, $url = null ) {
		$transports = array( 'curl', 'streams' );

		/**
		 * Filters which HTTP transports are available and in what order.
		 *
		 * @since 3.7.0
		 *
		 * @param string[] $transports Array of HTTP transports to check. Default array contains
		 *                             'curl' and 'streams', in that order.
		 * @param array    $args       HTTP request arguments.
		 * @param string   $url        The URL to request.
		 */
		$request_order = apply_filters( 'http_api_transports', $transports, $args, $url );

		// Loop over each transport on each HTTP request looking for one which will serve this request's needs.
		foreach ( $request_order as $transport ) {
			if ( in_array( $transport, $transports, true ) ) {
				$transport = ucfirst( $transport );
			}
			$class = 'WP_Http_' . $transport;

			// Check to see if this transport is a possibility, calls the transport statically.
			if ( ! call_user_func( array( $class, 'test' ), $args, $url ) ) {
				continue;
			}

			return $class;
		}

		return false;
	}