Extrapolate the original file from a digitally signed

[dacdam]

Hello everyone,
I'm trying to extrapolate the original file from a digitally signed.
I know that I have to use the function openssl_pkcs7_verify, but I can't manage to make it works, and I have already made several researches on the reason.

What I discovered is that the problem is because the digital signature that I'm trying to manage is in DER format, while PHP handles only PEM signatures.

Does anybody know if there is a way to convert a DER signature in a PEM signature? Keep in mind that the signature is attached to the file, then the encoding base64 I think is not possible as it would codify also the content of the file, making it unusable.

Alternatively, someone knows if there is a way to manage signatures DER in PHP?

Thanks for your help!