If you want to return te number of rows without fetching all data it might by more efficient to use this code (correct me if I'm wrong):
$sql_query = 'SELECT COUNT(*) AS NUMBER_OF_ROWS FROM (' . $your_query . ')';
$stmt= oci_parse($conn, $sql_query);
oci_define_by_name($stmt, 'NUMBER_OF_ROWS', $number_of_rows);
oci_execute($stmt);
oci_fetch($stmt);
echo $number_of_rows;