feat(php/s3/s3demo.php): verify policy & REST header strings before signing
feat(php/s3/s3demo.php): verify policy & REST header strings before signing

paul-herrmann/server-examples,paul-herrmann/server-examples,FineUploader/server-examples,paul-herrmann/server-examples,paul-herrmann/server-examples,FineUploader/server-examples,FineUploader/server-examples,FineUploader/server-examples,FineUploader/server-examples,FineUploader/server-examples,paul-herrmann/server-examples,paul-herrmann/server-examples,paul-herrmann/server-examples,FineUploader/server-examples,FineUploader/server-examples,paul-herrmann/server-examples