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Joomla 'Failed to start application: Failed to start the session': Fix

Symptoms: 'Error: Failed to start application' · 'Failed to start the session' · Often right after a PHP version change at the host

Mostly verified. Researched 2026-06-01 (kuneze, official forum threads, GitHub #22770). ✅ Verified in the Joomla 5.4.6 codebase: Factory.php throws “Failed to start application” and the session storage throws “Failed to start the session…” — so the combined message is real in current Joomla.

The site shows:

Error: Failed to start application: Failed to start the session

It means Joomla couldn’t initialise its session. The likely causes (the dominant one depends on your situation):

Cause 1 — Missing PHP mysqli extension (after a PHP version change)

Very common: your host switched/upgraded PHP and the new version doesn’t have the mysqli extension enabled. With a database session handler, Joomla then can’t start the session.

Fix: enable mysqli for the active PHP version (host PHP-extensions panel), or switch back to a PHP version that has it. Confirm under System → System Information → PHP Information (look for a mysqli section).

Cause 2 — Wrong database credentials in configuration.php

If the DB login is wrong, the session store can’t be reached. Verify $user, $password, $db, $host — see Could not connect to the database.

Cause 3 — Overcrowded #__session table

On sites using the Database session handler, a bloated/locked session table can cause this. Clearing expired rows from the #__session table (via phpMyAdmin) can help.

Cause 4 — Disk full

If the server’s disk is full, neither database nor filesystem sessions can be written. Check your hosting disk usage.

⚠️ A frequently-suggested “fix” is to switch the Session Handler from Database to Filesystem. That’s not a reliable primary fix and can mask the real cause (often the missing mysqli above) — diagnose the actual cause first.

joomla failed to start the session · joomla failed to start application session error · joomla session error after php upgrade · joomla #__session table full

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