get time and update row

I'm trying to do 2 things... My code works but I need it to record the time the data is saved so I can see if the data is stale also is there such thing as a insert or replace function or Update because I don't want to generate a new row every time but simply check if a user and group existent and update that row or if it doesn't then create the row... I know I use insert or replace in sqlite how about php --> mysql ?

<?php
$dbhost =xxxx
$dbuser = xxxx
$dbpass = xxxxx;
$dbname=xxxxx;


$con = mysql_connect("$dbhost","$dbuser","$dbpass");
mysql_select_db($dbname);

if (!$con)

  {
  die('Could not connect: ' . mysql_error());
  }


$sql="INSERT INTO gpscoordinates (UserName, Password1, Group1,Latitude,Longitude)
VALUES
('$_POST[User]','$_POST[Password1]','$_POST[Group1]','$_POST[Latitude]','$_POST[Longitude]')";

//$sql="INSERT INTO gpscoordinates (UserName, Password1, Group1,Latitude,Longitude)
//VALUES ('1','2','3','4','5')";


if (!mysql_query($sql,$con))
  {
  die('Error: ' . mysql_error());
  }
echo "1 record added";

mysql_close($con)
?> 

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