Php don't send empty value - php

Question

I've created a code that updates an Sql entry, but when you leave an value blank, it removes the existing value.
<?php
$servername = "localhost";
$username = "root";
$password = "HahahNo";
$dbname = "DATA";
$id = $_POST["id"];
$firstname = $_POST["firstname"];
$lastname = $_POST["lastname"];
$email = $_POST["email"];
// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
$sql = "UPDATE MyGuests SET firstname='$firstname', lastname='$lastname', email='$email' WHERE id=$id";
if ($conn->query($sql) === TRUE) {
echo "Success";
} else {
echo "Error updating record: " . $conn->error;
}
?>
Is there any way not to clear the not filled in values? Im fairly new to php.

Answer 1

Check if $_POST contains values.
First check if $_POST value has been set with isset() (has been posted).
Then check if the string isn't empty with strlen():
if (!isset($_POST['firstname'] || strlen($_POST['firstname']) == 0) {
echo 'Please enter first name';
} else {
// Do your DB update
}
Edit: Naturally empty() is better because you only need one check to do:
if (empty($_POST['firstname']) {
echo 'Please enter first name';
} ...
Thanks to #Fred -ii-
Edit II: As pointed out by #alex
if ($_POST['firstname']) {
echo 'Please enter first name';
} ...
would work too. Explained in PHP type comparison tables

Answer 2

You can write condition before execute function,
//all values are not null then update is perform
if(!is_null($firstname) && !is_null($lastname) && !is_null($email)){
if ($conn->query($sql) === TRUE) {
echo "Success";
} else {
echo "Error updating record: " . $conn->error;
}
}

Answer 3

What you're looking for is validation of user-supplied data. You're right - a user who hasn't read the instructions of your form could cause unintended data loss. Additionally a malicious user or innocent child could cause more harm with SQL injection.
First check your field meets minimum requirements. You could validate email addresses, postcodes, phone numbers, URLs... but to answer your question Daenu's answer is perfect.
Once you are satisfied the data meets your databases' requirements, check it is safe. Strip it of any malicious SQL commands. You can use prepared statements with PDO or MySQLi.
Also think about the specification of the data. Is the data type of each field a string or integer or float? What character set is your web page in (UTF-8 is a safe choice) and therefore what character set does your data arrive with? Does your database match (UTF8MB4 is a good companion)?