Pass variables with url in php as I am integrating

Question

 $url = 'https.secure.payu.in/_payment';

I want to pass the variables to the that site. like merchant key and other variables values. How I have to approach this.

And values I have passed like this

$post_values = array(
    "key" => pmpro_getOption("payu_merchantid")
);

as of now I am passing the URL Like this

wp_redirect($url);

Please give me proper idea for this.