Encode a string to have only letters and numbers for output

I need a PHP function that doesn't require any fancy server configuration that can encode a string, for instance an email address, where the output is ONLY letters and numbers. I've tried several examples I've found and some seem to put the "=" equals sign in the output which is causing me problems. I just don't want people seeing this to recognize the string as an email address. It isn't supposed to be secure, just a simple way to disguise the string. Any Ideas? I've tried base64_encode() and convert_uuencode() and from time to time each will include the "=" character.

Thanks.

Todd