Problem with processing multiple inbound webhooks in succession

[billsinc]

I'm trying to process inbound webhoks from a subscription provider. The problem I'm having is that they come in very fast. So for instance I'm unable to enable a new account because the account is not even created by the time the subscription hook comes in. I tried sleep(); before processing the subscription notification but that doesn't seem to work. I would appreciate some direction on how to handle this including the creation of a webhook queue if that's the best solution. Thanks!

// If new account, enter information into database 		
	if ($notification_type == "new_account_notification") {
	$insert = "INSERT INTO users (accountcode,firstname, lastname, email, username, password, companyname, approved)
	 			VALUES ('".$account_code."','".$first_name."','".$last_name."','".$email."','".$email."', '".$user_password_plain."','".$company_name."','0')";
	 $add_member = mysql_query($insert);		 
	 }

	// If subscription, sleep for X seconds to wait for account creation then approve account 
	if ($notification_type == "new_subscription_notification" || $notification_type == "reactivated_account_notification") {
		//sleep(15);
		$check = mysql_query("SELECT accountcode FROM users WHERE accountcode = '$account_code'") 
		 or die(mysql_error());
		 $check2 = mysql_num_rows($check);

		 // If the user exists, set approved=1
		 if ($check2 != 0) {
		 		$update_approved = mysql_query("UPDATE users SET approved=1 WHERE accountcode = '$account_code'")
		 		or die(mysql_error());
		}