kill

In this article, we will focus on the mysqli_kill() function in PHP, which asks the MySQL server to terminate a client connection identified by its thread (connection) ID. We will cover its syntax, parameters, both calling styles, and the real situations where killing a connection makes sense.

What mysqli_kill() does

mysqli_kill() is a built-in PHP function that sends a KILL command to the MySQL server for a given thread ID — the numeric identifier MySQL assigns to every client connection. It is the programmatic equivalent of running KILL <id>; in the MySQL shell.

A few points worth understanding before you use it:

• The thread ID you pass must belong to a connection on the same MySQL server. You typically obtain it with mysqli_thread_id().
• Killing a connection terminates it on the server side, but the local PHP mysqli object stays in scope. You should still call mysqli_close() to release it cleanly.
• Your database user needs the SUPER (or CONNECTION_ADMIN) privilege to kill connections it does not own.

When would you use it?

You rarely kill your own connection — closing it with mysqli_close() is simpler. mysqli_kill() earns its place when you have more than one connection and need to terminate a different one:

• A long-running query on another connection is holding locks and must be stopped.
• A monitoring or admin script needs to drop a stale or runaway connection by ID.
• You want to free a specific session after detecting unexpected or unauthorized activity.

Syntax and parameters

mysqli_kill() works in both the procedural and object-oriented MySQLi styles.

// Procedural
mysqli_kill(mysqli $mysql, int $thread_id): bool

// Object-oriented
$mysqli->kill(int $thread_id): bool

Parameters:

• mysql — A MySQLi connection object (procedural style only).
• thread_id — The connection/thread ID to terminate.

Return value: Returns true on success, false on failure.

Procedural example

The example below opens a connection, looks up its own thread ID, kills it, and then closes the handle:

<?php
$mysqli = mysqli_connect("localhost", "username", "password", "database");

if (!$mysqli) {
    die("Connection failed: " . mysqli_connect_error());
}

$thread_id = mysqli_thread_id($mysqli);

if (mysqli_kill($mysqli, $thread_id)) {
    echo "Connection {$thread_id} was killed on the server.";
}

mysqli_close($mysqli);
?>

Here we connect with mysqli_connect(), check the result to avoid fatal errors on later calls (see mysqli_connect_error()), read the current thread ID with mysqli_thread_id(), and pass it to mysqli_kill(). After the kill, the server-side connection is gone, so any further mysqli_query() on this handle would fail — we just close it.

Object-oriented example

The same logic in OOP style, killing a second connection from the first — the common real-world pattern:

<?php
$admin  = new mysqli("localhost", "username", "password", "database");
$worker = new mysqli("localhost", "username", "password", "database");

if ($admin->connect_errno || $worker->connect_errno) {
    die("Connection failed.");
}

// Suppose $worker is running something we need to stop.
$worker_id = $worker->thread_id;

if ($admin->kill($worker_id)) {
    echo "Killed worker connection {$worker_id}.";
}

$admin->close();
$worker->close();
?>

Conclusion

mysqli_kill() asks the MySQL server to terminate a connection by its thread ID, returning true on success and false on failure. Its real value shows up when an additional connection must be stopped — to release locks, drop a runaway query, or end a suspicious session — while a plain mysqli_close() remains the right tool for ending a connection you own.

Practice