this will split a string after a certain number of chararters - there are other ways of spliting string so try searching workwrap() or split() and you should find something that could help.
<?php
$text = "25637"; //zip code entered
$newtext = wordwrap($text, 3, " ", 1); //split the text after 3 characters - seporate with a space
echo "$newtext"; //display the message
?>
this will display "256 37" tho which i think is not quite want you want.