Both have more scopes. But you should keep in mind that with new Google updates, SEO has been totally changed. Many SEOs are suffreing form them because they don't know what to do to recover. And as for PHP, then i must say that the demand of PHP developer will never decrease. If you have good writing skills then you can go with SEO or SMO and if you have good analytical/programming skills then you can go with php developer.
However, my personal comment is learn the basics of SEO and then go with PHP development. This will help you to boost your future.