openssl_pkcs7_sign() takes the contents of the file named infilename and signs them using the certificate and its matching private key specified by signcert and privkey parameters.
headers is an array of headers that will be prepended to the data after it has been signed (see openssl_pkcs7_encrypt() for more information about the format of this parameter.
flags can be used to alter the output - see PKCS7 constants - if not specified, it defaults to PKCS7_DETACHED.
extracerts specifies the name of a file containing a bunch of extra certificates to include in the signature which can for example be used to help the recipient to verify the certificate that you used.
// the message you want to sign so that recipient can be sure it was you that
// sent it
$data = <<<EOD
You have my authorization to spend $10,000 on dinner expenses.
// save message to file
$fp = fopen("msg.txt", "w");
// encrypt it
if (openssl_pkcs7_sign("msg.txt", "signed.txt", "mycert.pem",
array("To" => "firstname.lastname@example.org", // keyed syntax
"From: HQ <email@example.com>", // indexed syntax
"Subject" => "Eyes only")
// message signed - send it!
exec(ini_get("sendmail_path") . " < signed.txt");