openssl_pkcs7_encrypt is returning a different Content-Transfer-Encoding header

Question

I am trying to encrypt some data with the next line of code:

openssl_pkcs7_encrypt($data, $temp, (array) $cert, ['Content-Transfer-Encoding: binary'], PKCS7_BINARY, $cipher

however, when I am receiving the result, I am getting the headers as an array:

[
   0 = "binary",
   1 = "base64"
]