PHP doesn't execute functions with sql correct - php

Question

I will put my code below. I basically check on value in the database and if it's 1 or 0 i want to change it to the opposite (so if 1 change it to 0, if 0 change to 1).
If I execute one SQL statement without using the function (but then it only works one way once) it works. But if I want to execute the specific function with it depending on what the value currently is, it doesn't seem to work. Do you know what I am doing wrong here?
<?php
$date_id = $_POST['dateID'];
$con = mysqli_connect("localhost","root","","secret_name");
$sql = "SELECT * FROM date_list WHERE date_id = ".$dateID;
$result = mysqli_query($con, $sql);
while($row = mysqli_fetch_assoc($result)){
$occupied = $row['occupied'];
if($occupied == 1){
decross_entry();
} elseif( $occupied == 0){
cross_entry();
}else{
echo "Error"
}
}
function decross_entry(){
$dateID = $_POST['dateID'];
$con_2 = mysqli_connect("localhost","root","","secret_name");
$sql_edit = "UPDATE date_list SET occupied= '0' WHERE date_id = ".$dateID;
if($con_2 -> connect_errno){
echo "Failed to connect to database!" . $con_2->connect_error;
}
if ($con_2 -> query($sql_edit) === TRUE)
{
echo "saved!";
} else {
echo "error: " .$sql_edit."<br>".$con_2->error;
}
}
function cross_entry(){
$dateID = $_POST['dateID'];
$con_2 = mysqli_connect("localhost","root","","secret_name");
$sql_edit = "UPDATE date_list SET occupied= '1' WHERE date_id = ".$dateID;
if($con_2 -> connect_errno){
echo "Failed to connect to database!" . $con_2->connect_error;
}
if ($con_2 -> query($sql_edit) === TRUE)
{
echo "saved!";
} else {
echo "error: " .$sql_edit."<br>".$con_2->error;
}
}
?>

Answer 1

If the only possible values of occupied are 0 and 1 then you can do what you want in one query without needing to look up the value of occupied first:
UPDATE date_list
SET occupied = 1 - occupied
WHERE date_id = ?
In PHP, using a prepared query to avoid SQL injection:
$date_id = $_POST['dateID'];
$con = mysqli_connect("localhost","root","","secret_name");
$sql = "UPDATE date_list SET occupied = 1 - occupied WHERE date_id = ?";
$stmt = $con->prepare($sql);
$stmt->bind_param('i', $date_id); // use 's' if $date_id is not an integer
$stmt->execute();