Okay, so depending on what the PHP script returns, the call could be something like this:
$.ajax({
url: 'ajax.php',
dataType: 'html',
data: {var1: 1}, // Data passed to the PHP function
success: function(results){
if(results = 1) {
$('#question123 input.radio:first').click();
}
else {
$('#question123 input.radio:last').click();
}
},
error: function(){
alert('Fail!');
}
});