To generate a self-signed certificate, pass NULL as the signing certificate (2nd parameter). For example:
$req_key = openssl_pkey_new();
$dn = array(
"countryName" => "US",
"stateOrProvinceName" => "Colorado",
"organizationName" => "yPass.net",
"organizationalUnitName" => "yPass.net",
"commonName" => "yPass.net Root Certificate"
);
$req_csr = openssl_csr_new($dn, $req_key);
$req_cert = openssl_csr_sign($req_csr, NULL, $req_key, 365);