OPENSSL_ZERO_PADDING

OPENSSL_ZERO_PADDING — By default encryption operations are padded using standard block padding and the padding is checked and removed when decrypting. If OPENSSL_ZERO_PADDING is set in the openssl_encrypt or openssl_decrypt $options then no padding is performed, the total amount of data encrypted or decrypted must then be a multiple of the block size or an error will occur.

OPENSSL_ZERO_PADDING = 2;