Ricardo,
I don't fully understand, but are you trying to access $usario from the function navegador() ?
If so, do this:
class Pagina {
var $usario;
function navegador() {
echo $this->usario;
}
}
$nueva_pagina = new Pagina();
$nueva_pagina->navegador();
basically, to access a class variable from a function inside the class, use $this->varname.
-sridhar