PHP prepare statement Notice: Only variables should be passed by reference - php

Question

my first time writing prepared statement
this code is checking your late in every 3 hours then inserting the time interval into db
date_default_timezone_set('Asia/Hong_Kong');
$now = new DateTime();
if ($now->format("H:i") > "22:00") {
$deadline = DateTime::createFromFormat("H:i", "22:00");
$diff = $now->diff($deadline);
echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "19:00") {
$deadline = DateTime::createFromFormat("H:i", "19:00");
$diff = $now->diff($deadline);
echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "16:00") {
$deadline = DateTime::createFromFormat("H:i", "16:00");
$diff = $now->diff($deadline);
echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "13:00") {
$deadline = DateTime::createFromFormat("H:i", "13:00");
$diff = $now->diff($deadline);
echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "10:00") {
$deadline = DateTime::createFromFormat("H:i", "10:00");
$diff = $now->diff($deadline);
echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "07:00") {
$deadline = DateTime::createFromFormat("H:i", "07:00");
$diff = $now->diff($deadline);
echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
}
$stmt = $conn->prepare("INSERT INTO time_in (e_id, login, late, date_in)
VALUES (?, ?, ?,CURRENT_TIMESTAMP)");
$stmt->bind_param("sss", $e_id, $login, $diff->format('%H:%i'));
$e_id = "id is unavailable"; // changing to $_POST in the future
$login = "1";
$status = $stmt->execute();
if(!$status) {
echo $stmt->error;
exit;
}
echo "success";
}
and im getting this
Notice: Only variables should be passed by reference
at this line $stmt->bind_param("sss", $e_id, $login, $diff->format('%H:%i'))
the code working perfectly but im getting that notice
please help

Answer 1

you must pass actual variables to bind_param, since $diff->format('%H:%i') is not a variable but rather the output of a function, it doesn't like that.
You will need to assign this to a variable first then pass it in. i.e
$diffFormat = $diff->format('%H:%i');
$stmt->bind_param("sss", $e_id, $login, $diffFormat);