Mysql NOW() is not updating the db - php

Question

I've following query which updated the db table parkings to column name : latitude, longitude, parking_time and let_parked. So It's update all the column but it's not updating the parking_time column with NOW(). Do you know why ?
if(isset($_POST["update"]) && $_POST["update"]==true)
{
if($userid == "1")
{
$parking_time = "(NOW() - INTERVAL 122 MINUTE)";
$results = $mysqli->query("UPDATE parkings SET latitude = '$mLat', longitude = '$mLng', parking_time = '$parking_time', let_parked = '1' WHERE locId = '$mId' ");
}
elseif($userid == '0')
{
$parking_time = "NOW() - INTERVAL 220 MINUTE";
$results = $mysqli->query("UPDATE parkings SET latitude = '$mLat', longitude = '$mLng', parking_time = '$parking_time' WHERE locId = '$mId' ");
}
if (!$results) {
//header('HTTP/1.1 500 Error: Could not Update Markers! $mId');
echo "coudld not update marker." . mysql_error();
exit();
}
exit("Updated successfully Done! $userid, $mId");
}

Answer 1

Remove the () from around your date value, e.g.
$parking_time = "(NOW() - INTERVAL 122 MINUTE)";
should be
$parking_time = "NOW() - INTERVAL 122 MINUTE";
and then remove the ' from around where you use that datetime:
UPDATE [snip], parking_time = $parking_time,[snip]
^-- ^-- no quotes
This will eventually produce
UPDATE ... parking_time = NOW() - INTERVAL 122 MINUTE, ....
instead of
UPDATE ... parking_time = '(NOW() - INTERVAL 122 MINUTE)' ...
Your version was generating a string which contained some text. But since that text was treated as a string, MySQL would NOT execute the NOW() - ... date math, so you were trying to literally set your db field value to an invalid date.