Make sure people use their Mine Craft username when checking out with Paid Memberships Pro

mc_isMinecraftUsername.php

/*
	Make sure people use their Minecraft username when registering.
*/
//uses Minecraft website to test a username and returns true or false
function mc_isMinecraftUsername($username, $force = false)
{
	global $premium_minecraft_usernames;
	
	if(!is_array($premium_minecraft_usernames))
		$premium_minecraft_usernames = array();

	if($force || !isset($premium_minecraft_usernames[$username]))
	{
		$premium = wp_remote_retrieve_body(wp_remote_get("https://minecraft.net/haspaid.jsp?user=" . $username));
		if($premium == "true")
			$premium_minecraft_usernames[$username] = true;
		else
			$premium_minecraft_usernames[$username] = false;
	}

	return $premium_minecraft_usernames[$username];
}

//check username at PMPro checkout
function mc_pmpro_registration_checks($continue)
{
	//is username even set?
	if(isset($_REQUEST['username']))
	{
		//okay check if it's a premium minecraft username
		if(!mc_isMinecraftUsername($_REQUEST['username']))
		{
			pmpro_setMessage("Your username must be a valid premium Mine Craft username.", "pmpro_error");
			return false;
		}
	}
	
	return $continue;
}
add_filter("pmpro_registration_checks", "mc_pmpro_registration_checks");