Try the following code. I'm sure it's not the only way of doing it but it does work. Hope it helps?
<?php
$numbers = "492";
$number1 = substr($numbers,0,1);
$number2 = substr($numbers,1,1);
$number3 = substr($numbers,2,1);
$number_array = array ("$number1", "$number2", "$number3");
sort($number_array);
$numbers_formatted = implode("", $number_array);
print "Numbers were: $numbers<br>";
print "Numbers are now: $numbers_formatted";
?>