Returns an x509 certificate resource on success, FALSE on failure.
openssl_csr_sign() generates an x509 certificate resource from the csr previously generated by openssl_csr_new(), but it can also be the path to a PEM encoded CSR when specified as file://path/to/csr or an exported string generated by openssl_csr_export(). The generated certificate will be signed by cacert. If cacert is NULL, the generated certificate will be a self-signed certificate. priv_key is the private key that corresponds to cacert. days specifies the length of time for which the generated certificate will be valid, in days.