`odbc_close()` does not report success and some drivers (namely Microsoft Access) don't seem to close connections at first attempt. This is normally not an issue, unless you need to establish many connections within the script lifetime.
You can use the fact that resource type changes (e.g. from "odbc link" to "Unknown") as a trick to figure out whether connection was successfully closed (and eventually retry):
<?php
$type = get_resource_type($conn);
$wait_until = time() + 3;
do {
odbc_close($conexion_odbc);
} while (get_resource_type($conn)===$type && time()<$wait_until);