Make directory with plugin activation

What am I doing wrong? I am writing a plugin that will save text files to a directory inside the plugin folder. When the plugin activates, I want it to check if the folder exists and, if not, create it and set permissions to 777.

I am doing a global define of the folder to use throughout the plugin.

The plugin activates, but no folder is created…

Thanks!

define('TXTFOLDER', get_bloginfo('wpurl')."/wp-content/plugins/fooplugin/txtfiles");

function foobar_install () {
	if (!file_exists(TXTFOLDER)) {
		mkdir(TXTFOLDER, 0777);
	}
}

register_activation_hook(__FILE__, 'foobar_install');