How to insert value to php code?

Question

Hi,

I have a PHP code which can check if in  MySQL database(which is on my host) exists username which  inserted  customer.In my C# .net application i have textbox and i want to give the textbox value to php code which will check if username is exists in database.if yes messageBox.Show("Yes") if no msgBox("no");

how can i do that?

Thanks