JavaScript - Rot13 Encrypting

Hey, I've got this script that aparantly encrypts plain text to rot13. Here it is:

Code:

		<script type="text/javascript">
        function rot13(txt) {
        
            var map = []
            var tmp = "abcdefghijklmnopqrstuvwxyz"
            var buf = ""
        
            for (j = 0; j < tmp.length; j++) {
                var x = tmp.charAt(j); var y = tmp.charAt((j + 13) % 26)
                map[x] = y; map[x.toUpperCase()] = y.toUpperCase()
            }
        
            for (j = 0; j < txt.length; j++) {
                var c = txt.charAt(j)
                buf += (c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z' ? map[c] : c)
            }
        
            return buf
        } 
        </script>
               
        <script>var foo = rot13('<?php rec9(); ?>')</script>
		<script>document.write(foo)</script>

I won't lie, my Javascript is terrible. I need to embed that script into a page, and have it encrpyt a PHP variable. I have no idea how because the script is assuming you're loading it externally. Can somebody help reconstruct the script so I can easily parse a PHP variable into it, and then output the result?

I really do need help, I've spent quite some time attempting and I've got pretty much nowhere.

P.s. PHP Code:
<?php rec9(); ?>

rec9(); is a function in PHP which retrieves whatever the user posted in the textbox (we can just assume that code really mean "12345") - as you can see, I've attempted to parse it but I just don't know where to start. Hopefully I haven't scrambled it too much so somebody who knows Javascript can't help.

I really appreciate any help.