You can do this in JS, see example code:
It goes over the input fields and checks if they have values in order to enable the submit button
<input type="submit" id="register" value="Register" disabled="disabled" />
...
(function() {
$('form > input').keyup(function() {
var empty = false;
$('form > input').each(function() {
if ($(this).val() == '') {
empty = true;
}
});
if (empty) {
$('#register').attr('disabled', 'disabled');
} else {
$('#register').removeAttr('disabled');
}
});
})()
Cheers,
Edo