Fatal error: Call to undefined method mysqli_result::format() - php

Question

I know that the error says that format isn't a valid code, but I used it before and it worked fine. So I hope someone knows a code to replace it with or now how to fix it.
require_once 'config.php';
$checker = "SELECT StartTime FROM ap21_Teachers WHERE Mentor_Class = 'I2C'";
$checker2 = "SELECT EndTime FROM ap21_Teachers WHERE Mentor_Class = 'I2C'";
$checker3 = "SELECT Minutes FROM ap21_Teachers WHERE Mentor_Class = 'I2C'";
$result1 = mysqli_query($mysqli, $checker);
$result2 = mysqli_query($mysqli, $checker2);
$minutes = mysqli_query($mysqli, $checker3);
$starttime = new DateTime($result1);
$endtime = new DateTime($result2);
while($starttime <= $endtime)
{
echo "<option value='".$starttime->format('H:i:s')."'>".$starttime->format('H:i:s')."</option>";
$starttime = date_add($starttime, date_interval_create_from_date_string($minutes, ' min'));
}
echo " </select>";
and above I grab the vars out of the database

Answer 1

The problem is mysqli_query returns a mysqli_result which you need to read before using:
require_once 'config.php';
$checker = "SELECT StartTime, EndTime, Minutes FROM ap21_Teachers WHERE Mentor_Class = 'I2C'";
$result = mysqli_query($mysqli, $checker); //Execute query
if (!$result) { //Result may be false if there's an error
echo "Error getting result";
}
$row = mysqli_fetch_assoc($result); //Put first result in an array
if (!$row) { // $row will be false if there's no result
echo "No data found";
}
//Notice how the array entry keys match the columns in the query.
$starttime = new DateTime($row["StartTime"]);
$endtime = new DateTime($row["EndTime"]);
$minutes = $row["Minutes"];
while($starttime <= $endtime)
{
echo "<option value='".$starttime->format('H:i:s')."'>".$starttime->format('H:i:s')."</option>";
$starttime = date_add($starttime, date_interval_create_from_date_string($minutes, ' min'));
}
echo " </select>";