How can I obfuscate or minify my php code?

One option that doesn't take much effort is to use this little script:

<?

$f = php_strip_whitespace ("common.php ");
file_put_contents("common_min.php ",$f);
?>

where "common.php " is the input file and "common_min.php " is the minified version of the file. 

This does not encrypt your code, and someone with some expertise can undo this with some work, but it does discourage modifications.  It also makes the code smaller if you want to keep down the size of the file.