From manual page: https://php.net/openssl.constants.other

If OPENSSL_RAW_DATA is set in the openssl_encrypt() or openssl_decrypt(), the returned data is returned as-is. When it is not specified, Base64 encoded data is returned to the caller.

For openssl_decrypt(), OPENSSL_RAW_DATA does NOT affect the representation of the returned data, but is used to indicate whether "data" (i.e., the ciphertext) is raw or base64-encoded. The function always returns the "plaintext" as-is.