Hi, I need to send 1 variable to windows, conecting the arduino to the pc from the usb port.
Now, this data will be put into a table I have in (lets say) port 81 of phpmyadmin.
How can I do this without a ethernet shield, since they are very hard to find in my country.
I need to send 1 variable to windows
You can NOT send a variable. You can send the data IN a variable.
You can't send anything to Windows. Windows is an operating system that doesn't know squat about serial ports.
You can send data to an application running on a Windows operating system. What application is it that you want to send data to? Does that application connect to a serial port? That is the only way that the Arduino can send the application data.
Now, this data will be put into a table I have in (lets say) port 81 of phpmyadmin.
This is nonsense. Tables are not in ports.
phpmyadmin is a PHP script that creates, manipulates, and deletes databases, tables, users, etc. It does NOT listen to serial.