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.