Delete alert php

So I want to make a website where when you delete a post it will show an alert msg that the post has been deleted successfully but the problem is that when I am reloading the alert is still there

Can't figure any fix for the issue if I try to use header('location: ./wb_submission.php'); the echo will not load

code I am trying ->

    if (isset($_GET['delete'])) {
        $sno = $_GET['delete'];
        $sql = "DELETE FROM `website_submit` WHERE `sno` = $sno";
        $result = mysqli_query($conn, $sql);
        echo '<div class="alert alert-danger" role="alert">
        The submission was deleted successfully!
      </div>';
    }

You should remove delete parameter from URL after page loaded. You can do it with js:

window.history.replaceState({}, '', location.href.replace(location.search, ""));

How it works

1. this method modifies the current URL without putting this into history

window.history.replaceState

2. First parameter: {}

The state object is a JavaScript object which is associated with the history entry passed to the replaceState method. The state object can be null.

3. Second parameter: ''

This parameter exists for historical reasons, and cannot be omitted; passing the empty string is traditional, and safe against future changes to the method.

4. Third parameter (new URL): first part (location.href) is complete URL value, like:
   http://example.com/page.php?delete=10
   then it will replace search parameters with empty string:

.replace(location.search, "")

location.search is everything after ? in URL (in this case: ?delete=10)

History.replaceState()

Edit

If you have multiple parameters in your URL and you want to remove only delete parameter try this code:

function removeDeleteParam(){

   // Create new URLSearchParams object
   // location.search is something like "?delete=10&msg=hello&ok=true"
   let params = new URLSearchParams(location.search);

   // Remove delete parameter from URL parameters
   params.delete("delete");

   // Convert parameters to string
   // e.g "msg=hello&ok=true"
   let urlParams = params.toString();

   // if urlParams is not empty add a question mark before it:
   urlParams = urlParams.length > 0 ? "?" + urlParams : "";

   // Replace new parameters
   window.history.replaceState({}, '', urlParams);

}

removeDeleteParam();

Edit 2 - Server-side redirect

A better approach is to redirect to a new URL directly from server using Location header, before redirecting you need to remove 'delete' parameter, also for success message you can add another parameter to show a message. Here is the idea:

if( isset( $_GET["delete"] ) ) {

    $id = $_GET["delete"];

    // Delete actions here, for example:
    $success = delete_item( $id );

    // You need to get the full URL, e.g: "https://example.com/admin/?delete=10"
    $full_url = get_current_url();
    
    // Then you have to remove the 'delete' parameter
    $new_url = remove_url_param( $full_url, "delete" );
    
    // Now add another parameter for deletion state (success or failure)
    if( $success ){
        $new_url = add_url_param( $new_url, "delete-success" );
    }
    else{
        $new_url = add_url_param( $new_url, "delete-failure" ); 
    }
    
    // Now redirect it
    header( "Location: " . $new_url );

}

Also for more security, you should consider casting the ID to integer, sanitize the URLs, etc. See the full example:

functions.php:

function delete_item( $id ){

    // Be careful, id is already a number here, but can be used for SQL injection
    // Always use mysqli_prepare before executing it

    // Check if it's really a number and it's positive
    $id = intval( $id );
    if( $id <= 0 )
       return false;

    $sql = "DELETE FROM `website_submit` WHERE `sno` = $id";

    // Execute the query
    // $result = mysqli_query( $conn, $sql );

    // Check here if it was successful and return it
    // return $success;

    // Just for test, a random state
    return rand( 1, 9999 ) % 2 == 0;

}

function get_current_url() {

    // Use HTTPS if set and not off
    $scheme = ( !empty( $_SERVER["HTTPS"] ) && $_SERVER["HTTPS"] !== "off" ) ? "https" : "http";

    // Get host and port
    $host = $_SERVER["HTTP_HOST"];

    // Get full request URI (path + query string)
    $request_uri = $_SERVER["REQUEST_URI"];

    // Build and sanitize
    $url = $scheme . "://" . $host . $request_uri;

    // Sanitize (remove invalid characters) to get a safe URL
    return filter_var( $url, FILTER_SANITIZE_URL );

}

function build_url( $parts ) {

    // HTTP or HTTPS
    $url = isset( $parts["scheme"] ) ? "{$parts['scheme']}://" : '';

    // Host URL
    $url .= $parts['host'] ?? '';

    // URL path
    $url .= $parts['path'] ?? '';

    // Query parameters
    $url .= isset( $parts["query"] ) ? "?{$parts['query']}" : '';

    return $url;

}


function add_url_param( $url, $key, $value ) {

    $parts = parse_url( $url );

    // Parse query string if exists
    $query = [];
    if( !empty( $parts["query"] ) ) {
        parse_str( $parts["query"], $query );
    }

    // Add or update the key
    $query[$key] = $value;

    // Rebuild query string
    $parts["query"] = http_build_query( $query );

    // Reconstruct URL
    return build_url( $parts );

}

page.php:

if( isset( $_GET["delete"] ) ){

    // Accepts only numbers
    $id = intval( $_GET["delete"] );

    // Check if it was deleted
    $success = delete_item( $id );

    // Get the full URL
    $full_url = get_current_url();

    // Remove 'delete' param
    $new_url = remove_url_param( $full_url, "delete" );

    // Add 'delete-success' or 'delete-failed'
    $new_url = add_url_param( $new_url, $success ? "delete-success" : "delete-failed", $id );

    // Redirect
    header( "Location: $new_url" );

}

if( !empty( $_GET["delete-success"] ) ){
    $id = intval( $_GET["delete-success"] );
    echo "Item $id was deleted.";
}
else if( !empty( $_GET["delete-failed"] ) ){
    $id = intval( $_GET["delete-failed"] );
    echo "Could not delete $id item.";
}