No error message in this code but it doesn't delete the fields

Question

In this code I don't have any error messages but it doesn't delete the data. By the way the database has many records and the names of all the fields are correct.

<?php

if ($connect = mysqli_connect('localhost', 'root', 'adminpass', 'flip'))
    {
    $id = $_GET['id'];
    $sql = "SELECT * FROM threads ORDER BY id DESC";
    $query = mysqli_query($connect, $sql);
    $num = mysqli_num_rows($query);
    }

?>

    <form action="<?php echo $_SERVER["PHP_SELF"]; ?>" method="POST" >

    <table border="1" width="400" cellpadding="0" cellspacing="0">
    <tr>
    <td>#</td>
    <td>id</td>
    <td>subject</td>
    </tr>

    <?php

while ($row = mysqli_fetch_array($query))
    {
?>

    <tr>
    <td> <input type="checkbox" name="checkbox[]" value="<?php echo $row['id'] ?>"></td>
    <td><?php echo $row['id'] ?></td>
    <td><?php echo $row['topic'] ?></td>
    </tr>   

<?php
    }
?>
    <input type="submit" name="delete" value="delete" >

<?php

if (isset($delete))
    {
    for ($i = 0; $i < $num; $i++)
        {
        $del_id = $checkbox[$i];
        $sql2 = "DELETE FROM threads WHERE id='$del_id'";
        $query2 = mysqli_query($connect, $sql2);
        }

    if ($query2)
        {
        echo "<meta http-equiv=\"refresh\" content=\"0;URL=delete.php\">";
        }
    }

mysqli_close($connect);
?>
</table>

</form>

What is the problem with it?

Answer 1

You can try this:

<?php

/* ESTABLISH CONNECTION */

$connect=mysqli_connect("localhost","root","adminpass","flip");

if(mysqli_connect_errno()){

echo "Error".mysqli_connect_error();
}

if(isset($_POST['delete'])){ /* IF DELETE IS CLICKED */

$checkbox=$_POST['checkbox'];
$hiddencounter=mysqli_real_escape_string($connect,$_POST['hiddencounter']); /* PREVENT A BIT OF SQL INJECTION */

for($i=0;$i<$hiddencounter;$i++){ /* FOR LOOP BASED ON THE NUMROWS BELOW */

   if(!empty($checkbox[$i])){ /* IF THE CHECKBOX IS TICKED */
        $del_id=mysqli_real_escape_string($connect,$checkbox[$i]);  /* PREVENT A BIT OF SQL INJECTION */
        $sql2="DELETE FROM threads WHERE id='$del_id'";
        $query2=mysqli_query($connect,$sql2); /* IMPLEMENT THE DELETE QUERY */

   }

} /* END OF FOR LOOP */

echo "<meta http-equiv=\"refresh\" content=\"0;URL=delete.php\">";

} /* END OF ISSET DELETE */

?>

<form action="" method="POST" > <?php /* YOU CAN LEAVE THE ACTION BLANK, TO SUBMIT THE FORM ON THE PAGE ITSELF */ ?>

<table border="1" width="400" cellpadding="0" cellspacing="0">
<tr>
<td>#</td>
<td>id</td>
<td>subject</td>
</tr>

<?

$sql="SELECT * FROM threads ORDER BY id DESC";
$query=mysqli_query($connect,$sql);
$num=mysqli_num_rows($query);

/* SUBMIT THE NUMBER OF ROWS QUERIED THROUGH HIDDEN INPUT */
echo "<input type='hidden' name='hiddencounter' value='$num'>";

while($row=mysqli_fetch_array($query)){ /* FETCH DATA */

?>

<tr>
<td><input type="checkbox" name="checkbox[]" value="<?php echo $row['id'] ?>"></td>
<td><?php echo $row['id'] ?></td>
<td><?php echo $row['topic'] ?></td>

</tr>   

<?
} /* END OF WHILE LOOP $QUERY */
?>

<input type="submit" name="delete" value="delete" >

<?php

mysqli_close($connect);

?>
</table>

</form>

Explanation why your code did not work:

• Your isset($delete): where does $delete variable came from?
• Your for loop doesn't determine if the checkboxes submitted has values or not.
• Your <meta> redirect. It redirects to delete.php after the for loop, so that is the reason you don't see any errors. But I'm pretty sure you would see some errors if you remove the <meta> redirect.
• Even if the delete is working, you wouldn't see the result right after because the delete query is after your fetching of data.

Answer 2

You have an error in your Variable:

if (isset($delete))

Should be:

   if (isset($_POST['$delete']))

Answer 3

I don't see where $delete variable is ever set in your code. At the moment $delete is not set, therefor if(isset($delete)) is false.

To me it would appear you are trying to use Registered Globals. Or you just forgot to set $delete to one of your $_GET[] or $_POST[] variables.

"Warning This feature has been DEPRECATED as of PHP 5.3.0 and REMOVED as of PHP 5.4.0."

PHP manual - Security Globals

Predefined Variables