The function will generate an array of integers even if your numerical parameters are enclosed in quotes.
<?php
var_dump( range('1', '2') ); // outputs array(2) { [0]=> int(1) [1]=> int(2) }
?>
An easy way to get an array of strings is to map strval() to the range:
<?php
var_dump( array_map('strval', range('1', '2')) ); // outputs array(2) { [0]=> string(1) "1" [1]=> string(1) "2" }
?>