“ini_set(): Cannot change zlib.output_compression – headers already sent”

Resolved

(@syzygist)

The weekly backup job on one of the sites I manage suddenly started showing a warning during weekly backups created after 3/4/26. The log is below, showing the warning, which appeared on both the 3/11/26 and 3/18/26 backups.

No changes were made to the job in this timeframe. The only things I changed between 3/4/26, when the job last ran without the warning, and 3/11/26, when it first ran with the warning, were:

3/5/26 – BackWPup 5.6.5 > 5.6.6

3/9/26 – Burst Statistics 3.2.0 > 3.2.1 (I applied a subsequent update to 3.2.3 on 3/12/26, warnings recurred on the 3/18/26 backup)

3/10/26 – WordPress core – 6.9.1 > 6.9.3

Both the zip and zliib PHP extensions are enabled (PHP 8.3).

[INFO] BackWPup 5.6.6; A project of WP Media
[INFO] WordPress 6.9.4 on
[INFO] Log Level: Normal
[INFO] BackWPup job: Weekly Full Site Backup
[INFO] Logfile is: backwpup_log_2026-03-18_01-36-37.html
[INFO] Backup file is: 2026-03-18_01-36_backwpup_DBDUMP-FILE-WPPLUGIN.zip
[18-Mar-2026 01:36:37] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:36:37] 1. Try to backup database …
[18-Mar-2026 01:36:37] Connected to database on localhost
[18-Mar-2026 01:36:39] Added database dump with 32.45 MB to backup file list
[18-Mar-2026 01:36:39] Database backup done!
[18-Mar-2026 01:36:39] 1. Trying to make a list of folders to back up …
[18-Mar-2026 01:36:40] Added "wp-config.php" to backup file list
[18-Mar-2026 01:36:40] 1391 folders to backup.
[18-Mar-2026 01:36:41] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:36:41] 1. Trying to generate a file with installed plugin names …
[18-Mar-2026 01:36:41] Added plugin list file "_.pluginlist.2026-03-18.txt" with 2.30 KB to backup file list.
[18-Mar-2026 01:36:41] 1. Trying to generate a manifest file …
[18-Mar-2026 01:36:41] Added manifest.json file with 2.85 KB to backup file list.
[18-Mar-2026 01:36:41] 1. Trying to create backup archive …
[18-Mar-2026 01:36:41] Compressing files as ZipArchive. Please be patient, this may take a moment.
[18-Mar-2026 01:37:08] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:37:44] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:38:03] Backup archive created.
[18-Mar-2026 01:38:03] Archive size is 717.35 MB.
[18-Mar-2026 01:38:03] 9213 Files with 896.06 MB in Archive.
[18-Mar-2026 01:38:04] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:38:04] 1 old log deleted
[18-Mar-2026 01:38:04] WARNING: Job finished with warnings in 87 seconds. Please resolve them for correct execution.

Any thoughts about what the problem might be? I manage many sites (in separate accounts) on this host, and am not seeing this warning anywhere else.

Viewing 5 replies - 1 through 5 (of 5 total)

Plugin Support Saransh
(@saranshwpm)

5 months, 3 weeks ago
Hi,

Thank you for sharing this.

We are currently investigating similar reports, so this is helpful.

In your case, could you please try the following and let me know how it goes:
- Temporarily deactivate the Burst Statistics plugin and check if the warning still appears
- If possible, try disabling zlib/output compression on your server and see if that makes any difference
This will help us narrow down what might be causing the issue on your setup.

Looking forward to your update, and we’ll continue from there.

Best regards,
Thread Starter syzygist
(@syzygist)

5 months, 3 weeks ago

I deactivated Burst Statistics and ran the backup job again – same errors. How would I disable zlib/output compression? The site is in a cPanel account.

Plugin Support Saransh
(@saranshwpm)

5 months, 2 weeks ago

Hi @syzygist,

Thank you again for reporting this and for sharing all the details earlier, it really helped.

I wanted to let you know that our developers have reviewed this and confirmed the issue. A fix is already planned and will be included in one of the upcoming releases.

We truly appreciate your patience while we work on this, and thanks again for bringing it to our attention.

If you notice anything else or have any questions in the meantime, feel free to let me know.

Best regards,

Thread Starter syzygist
(@syzygist)

4 months, 3 weeks ago

Confirming that after the update to version 5.6.8, I no longer see these errors in the backup log.

Plugin Support Saransh
(@saranshwpm)

4 months, 3 weeks ago

Hi @syzygist,

Thank you for confirming, I’m glad to hear the update to 5.6.8 resolved the issue on your side.

And thank you as well for reporting it earlier, feedback like yours helps improve the plugin.

If you have a moment and feel your issue was resolved, we’d truly appreciate it if you would consider updating or leaving a review here:
https://wordpress.org/support/plugin/backwpup/reviews/#new-post

Thanks again for taking the time to follow up 👍

Viewing 5 replies - 1 through 5 (of 5 total)

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