Kode unix automatic in form input

kodeunix.php

<?php 

$conn = mysqli_connect( "localhost", "root", "", "no_urut" ) or die ( "Error !".mysqli_error( $conn ) );
// Date 
date_default_timezone_set( 'Asia/Jakarta' );
$date = date( 'Y' );
$get3number = substr( $date,-3 ); //get 3 number of years from right

// Random of the character
function random_char( $panjang ) { 
	$karakter= 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'; 
	$string = ''; 
	for ( $i = 0; $i < $panjang; $i++ ) { 
		$pos = rand( 0, strlen( $karakter ) - 1); 
		$string .= $karakter{$pos}; 
	} 
return $string;
}

// Get data from database
$get_data = mysqli_query( $conn, "SELECT *FROM kode" );

// Check
$check = mysqli_num_rows( $get_data );

// var_dump( $check ); -> to check variable
if ( empty( $check ) ){
	$kd = 1;
} else {
	$kd = $check + 1;
}
 
?>
 
<form action="save-kode.php" method="post">
Kode: <input type="text" name="automatic_number" value="<?php echo random_char(3) . $get3number . '-' . $kd;  ?>" readonly>
<input type="submit" name="save_kode" value="Save !">
</form>
 
 
<!-- File save-kode.php -->
<?php  

$conn = mysqli_connect( "localhost", "root", "", "no_urut" ) or die ( "Error !".mysqli_error( $conn ) );

$kode = mysqli_real_escape_string( $conn, $_POST['automatic_number'] );

$sql = mysqli_query( $conn, "INSERT INTO kode( kode_unik ) VALUES( '$kode' )" );

if ( $sql ) {
	echo "<script>alert( 'Berhasil disimpan !' ); window.location='./';</script>";
} else {
	echo "Error: " . $sql . "<br>" . mysqli_error( $conn );
}
?>