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What happens to an ordinary PHP connection?
A non-persistent connection normally lives as long as its PHP connection object. If the script reaches its end without an explicit close, PHP cleans up the connection automatically. The PHP Manual states: “If you don’t do this explicitly, PHP will automatically close the connection when your script ends.”
For PDO, destruction occurs only after the PDO object is no longer referenced. That includes references held by statements or other objects. Assigning the PDO variable to null can release it early, but any remaining references must also be removed.
For MySQLi, the connection is likewise released automatically when the script ends. The PHP Manual describes explicit closure as optional, while recommending it when database operations are complete and substantial processing remains.
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When should you close it explicitly?
Close after the last query when significant work remains
If the application has finished all database operations but will continue processing for a meaningful amount of time, close the ordinary connection at that point. This releases the connection before the request or command finishes instead of holding it during unrelated work.
$pdo = new PDO($dsn, $user, $password);
// Run all required queries here.
$rows = $pdo->query($sql)->fetchAll();
// Release the PDO object after database work is complete.
$pdo = null;
// Continue with CPU-heavy processing, file generation, or other non-DB work.
With MySQLi, call its close method after the final database operation:
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$mysqli = new mysqli($host, $user, $password, $database);
$result = $mysqli->query($sql);
// No more queries will use this connection.
$mysqli->close();
// Continue processing without the database connection.
Let normal cleanup handle short scripts
If database work runs near the end of the script, an explicit close usually adds no practical benefit. Allowing PHP to clean up at script termination is supported behavior for ordinary PDO and MySQLi connections.
Close only when ownership is clear
In applications built from several components, a connection may be passed to repositories, services, callbacks, or deferred jobs. Do not close it merely because one function has finished. Close it only when the part of the application responsible for the connection can safely establish that no later operation needs it. Otherwise, keep the connection available and rely on ordinary end-of-script cleanup.
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PDO and MySQLi: practical differences
| Connection type | End-of-script behavior | Early-close guidance |
|---|---|---|
| Ordinary PDO | Released when the PDO object is destroyed; PHP also cleans it up when the script ends. | Set the PDO reference to null after the final database operation if substantial processing remains. Remove other references, including statement references, when necessary. |
| Ordinary MySQLi | Released automatically at script end. | Call $mysqli->close() after the final database operation when the script will continue with substantial non-database work. |
| Persistent PDO or MySQLi | Retained for reuse rather than closed at script end. | Manage pool capacity and reset connection state deliberately; do not assume script termination ends the server-side session. |
Why persistent connections require extra care
A persistent connection is kept in a pool so a later request can reuse it. That can avoid establishing a new connection for every request, but it also means connections remain open beyond the lifetime of one script.
Capacity is a server and worker concern
Persistent connections can increase the number of open database connections. The practical limit depends on database connection limits, the number of PHP workers, and idle-timeout configuration. Raising a server’s maximum connection setting is not a universal fix; first determine how many workers and persistent sessions your deployment can support.
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Session state can leak into the next request
PDO does not automatically clean up every change made to a persistent connection before it is reused. Temporary tables, locks, open transactions, session variables, and other stateful changes can remain. Application code must finish or roll back transactions, release locks, remove temporary objects, and restore any connection settings that a later request should not inherit.
An explicit close() at the end of a script does not turn a persistent connection into an ordinary one. The important task is to leave the reusable session in a known, safe state and configure the pool within the database and PHP worker limits.
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A decision checklist
- Using an ordinary PDO or MySQLi connection? End-of-script cleanup is sufficient when no substantial work follows the final query.
- Does substantial non-database processing remain? Close the ordinary connection as soon as the last database operation is complete.
- Could another component issue a query later? Keep the connection open until ownership and query completion are certain.
- Using persistence? Plan for pooled capacity and explicitly clean up transactions, locks, temporary tables, and other session state.
- Debugging connection exhaustion? Check whether persistence, worker counts, server limits, or long-running work is keeping sessions open; do not assume that adding a close call to every script is the answer.
Common mistakes
Closing after every query
Opening and closing a connection around each individual query is unnecessary for a normal request and prevents later queries from using the same connection. Keep one connection for the unit of work, then close it when that unit is complete if early release is useful.
Assuming a persistent connection disappears at script end
It does not. Persistence is specifically designed to retain connections for reuse, so script termination is not a reliable signal that the database session has ended.
Dropping only the PDO variable
If a statement or another object still references the PDO connection, destruction may be delayed. Release those references as well when you need deterministic early cleanup.
Best-practice answer
For ordinary PHP database connections, explicit closure is a conditional optimization and a clarity choice, not a universal requirement. Finish all database work, close the connection before lengthy non-database processing, and otherwise let PHP clean it up at script termination. Treat persistent connections as a separate operational design: limit the pool appropriately and reset every piece of session state that must not survive into the next request.
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