( string filename, int flags [, string outfilename [, array cainfo [, string extracerts]]] )
openssl_pkcs7_verify() reads the S/MIME message
contained in the filename specified by filename and
examines the digital signature. It returns TRUE if the signature is
verified, FALSE if it is not correct (the message has been tampered with,
or the signing certificate is invalid), or -1 on error.
flags can be used to affect how the signature is
verified - see PKCS7 constants
for more information.
If the outfilename is specified, it should be a
string holding the name of a file into which the certificates of the
persons that signed the messages will be stored in PEM format.
If the cainfo is specified, it should hold
information about the trusted CA certificates to use in the verification
process - see certificate
verification for more information about this parameter.
If the extracerts is specified, it is the filename
of a file containing a bunch of certificates to use as untrusted CAs.