prevent insert same id if the user/student not put timeout - php

Question

i have two button on my homepage one is time-in and the other is time-out,
i want to prevent the user/student to time-in using same id if he did not put time-out on his last time-in to create valid entry. Hope you can help me.
here is my php code:
<?php
include_once('connection.php');
if(isset($_POST['submit0'])){
$rfid = $_POST['rfid'];
$time=date("H:i:s");
$sql = mysqli_query($conn, "SELECT * FROM stud WHERE rfid_num = '$rfid'");
$count = mysqli_num_rows($sql);
if ($count == 0 ) {
header("location:notexist.php");
} elseif (empty($row['timeout'])) {
header("location:page say the user/student need to put timeout first before time-in again");
} else {
while( $row = mysqli_fetch_array($sql)) {
$rfid=$row['rfid_num'];
$id=$row['id'];
$name0 = $row['name'];
$course0 = $row['course'];
$image = $row['image'];
$InsertSql = "INSERT INTO student_att(rfid_num,id,name,course,image,timein) VALUES ('$rfid','$id','$name0','$course0','$image','$time')";
$res = mysqli_query($conn, $InsertSql);
}
}
}
?>

Answer 1

this is my answer just wanna share it, i just add select student_att table
to fetch the data and check if timeout column is empty.
<?php
include_once('connection.php');
if(isset($_POST['submit0'])){
$rfid = $_POST['rfid'];
$time=date("H:i:s");
$sql = mysqli_query($conn,"select * from stud where rfid_num ='$rfid' ");
$count = mysqli_num_rows($sql);
if ($count == 0) {
header("location:notexist.php");
}else{
while( $row = mysqli_fetch_array($sql)) {
$rfid=$row['rfid_num'];
$id=$row['id'];
$name0 = $row['name'];
$course0 = $row['course'];
$image = $row['image'];
$sql1 = mysqli_query($conn,"select * from student_att where rfid_num ='$rfid' order by number DESC limit 1 ");
while( $row = mysqli_fetch_array($sql1)) {
if(empty($row['timeout'])){
header("location:logout.php");
}else{
$InsertSql = "INSERT INTO student_att(rfid_num,id,name,course,image,timein) VALUES ('$rfid','$id','$name0','$course0','$image','$time')";
$res = mysqli_query($conn, $InsertSql);
}
}
}
}
}
?>