PHP rot13 with ASCII

I want to make simple encryption with rot13, but i don't want use function str_rot13(), because I want add numbers on it and I think str_rot13() isn't supported. I have found code like this and want to edit, but I don't know how

<?php 

$string = "Hello World 123";

for ($i = 0, $j = strlen( $string); $i < $j; $i++) 
{
    // Get the ASCII character for the current character
    $char = ord( $string[$i]); 


    // If that character is in the range A-Z or a-z, add 13 to its ASCII value
    if( ($char >= 65  && $char <= 90) || ($char >= 97 && $char <= 122)) 
    {
        $char += 13; 

        // If we should have wrapped around the alphabet, subtract 26
        if( $char > 122 || ( $char > 90 && ord( $string[$i]) < 97)) 
        {
            $char -= 26;
        }
    }
    echo chr( $char);
}

 ?>

this code have result "Uryyb Jbeyq 123". What I want is add number 0-9 on it, so encryption become rot18 -> a-z0123456789. Result of this so become "Zw336 E693v JKL"