我刚找到！

$sql = "INSERT INTO MY_TABLE(id, signature)
        VALUES (1, EMPTY_CLOB())
        RETURNING signature INTO :signature";

$stid = oci_parse($conn, $sql);
$clob = oci_new_descriptor($conn, OCI_D_LOB);
oci_bind_by_name($stid, ":signature", $clob, -1, OCI_B_BLOB);
oci_execute($stid, OCI_NO_AUTO_COMMIT);
$clob->save(base64_decode($mySignatyure);

oci_commit($conn);

是工作。
一天的研究就为了这个。.