php session management using postgresql

sessions.inc.php

<?php
#
# manejador de sesiones
#
$sessiondb = pg_pconnect('dbname=sessions user=bob password=secret');

function on_session_start($save_path, $session_name) {
  error_log("on_session_start: " . $session_name . " ". session_id());
}

function on_session_end() {
  global $sessiondb;
  pg_close($sessiondb);
}

function on_session_read($key) {
  global $sessiondb;
  #error_log("on_session_read: " . $key);
  $stmt = "SELECT session_data FROM sessions WHERE session_id ='$key' AND CURRENT_TIMESTAMP < session_expiration";
  $sth = pg_query($sessiondb, $stmt);

  if($sth) {
    $row = pg_fetch_array($sth);
    return($row['session_data']);
  } else {
    return $sth;
  }
}

function on_session_write($key, $val) {
  global $sessiondb;
  #error_log("on_session_write $key = $val");
  $val = pg_escape_string($val);
  $result = pg_query($sessiondb, "SELECT session_id FROM sessions WHERE session_id='$key'");
  $existe = pg_num_rows($result);
  if ($existe === 1)
    $sql  = "UPDATE sessions SET session_data ='$val', session_expiration = CURRENT_TIMESTAMP + interval '1 hour' WHERE session_id ='$key'";
  else
    $sql  = "INSERT INTO sessions VALUES ('$key', '$val',CURRENT_TIMESTAMP + interval '1 hour')";
  pg_query($sessiondb, $sql);
}

function on_session_destroy($key) {
  global $sessiondb;
  pg_query($sessiondb, "DELETE FROM sessions WHERE session_id = '$key'");
}

function on_session_gc($max_lifetime) {
  global $sessiondb;
  pg_query($sessiondb, "DELETE FROM sessions WHERE session_expiration > CURRENT_TIMESTAMP");
}


# Set the save handlers
session_set_save_handler("on_session_start", "on_session_end", "on_session_read", "on_session_write", "on_session_destroy", "on_session_gc");
session_start();