How to deal with log messages?

In Piwigo you have a $logger object, available at any time in your code. By using it, the message will be added in the right file, at the right place. The logging system generates one file per day and (by default) keeps the 30 last days:

$ ll _data/logs/
total 280
-rw-rw-rw- 1 www-data www-data    12 Oct 19  2022 index.htm
-rw-r--r-- 1 www-data www-data  9316 Nov 10 16:22 log_2025-11-10_0b5695e16ff4cca0df6e332b469eb105d2bc6e71.txt
-rw-r--r-- 1 www-data www-data  9260 Nov 11 18:45 log_2025-11-11_82170d05069d936e59e8a293d1ff8e1d5f79b4d7.txt
-rw-r--r-- 1 www-data www-data   104 Nov 12 11:30 log_2025-11-12_761f3addaf7ad2c2fd71f1c45393e72df903d367.txt
-rw-r--r-- 1 www-data www-data   208 Nov 13 17:57 log_2025-11-13_919aee1f3dee54623c481aa3168fc7843db41c1b.txt
-rw-r--r-- 1 www-data www-data  1001 Nov 14 17:35 log_2025-11-14_d30be0f55d811b4bbab9e561c9368d1299313eec.txt
-rw-r--r-- 1 www-data www-data  2844 Nov 17 17:03 log_2025-11-17_fa93d9c73e8f8d7543b460a357522fbe1095adc1.txt
-rw-r--r-- 1 www-data www-data 14460 Nov 18 18:34 log_2025-11-18_e36eef127b01d951ba4b579259c29e8f15dc834e.txt
-rw-r--r-- 1 www-data www-data 28083 Nov 19 14:22 log_2025-11-19_29854df472a63f60e7645d372d9b4bc87af86a0e.txt
-rw-r--r-- 1 www-data www-data  3539 Nov 20 19:37 log_2025-11-20_f84f7e147daf80d6e26a87b564cc062c89988bef.txt
-rw-r--r-- 1 www-data www-data 55427 Nov 21 17:33 log_2025-11-21_bee5fdc5f3d1c7f729ae3832e5207b99b1a55c52.txt
-rw-r--r-- 1 www-data www-data   568 Nov 24 15:30 log_2025-11-24_2f61236df90467d8d092ecd80e57ea96d0ea8825.txt
-rw-r--r-- 1 www-data www-data 38241 Nov 25 17:01 log_2025-11-25_45148d65614ee51e285ac2ca7444ce381e212ce3.txt
-rw-r--r-- 1 www-data www-data  9921 Nov 27 12:25 log_2025-11-27_8346767718dbc7f3635b87d95e2c0c523629a17f.txt
-rw-r--r-- 1 www-data www-data 30303 Nov 28 23:49 log_2025-11-28_4f40b311d5fad41081a2def89696e1ad45816fd2.txt
-rw-r--r-- 1 www-data www-data  2329 Nov 29 18:46 log_2025-11-29_61f2b6d759239a066c972c47cd8b15c2047d9ec0.txt
-rw-r--r-- 1 www-data www-data  4171 Dec  1 15:05 log_2025-12-01_e3c77e81151926c3db20bba077e98ec96fb2d17f.txt
-rw-r--r-- 1 www-data www-data  2079 Dec  2 14:46 log_2025-12-02_00d86aab5e3b92f0504eb27c32b600332273f783.txt
-rw-r--r-- 1 www-data www-data   104 Dec  3 11:57 log_2025-12-03_b8a9e0615e298878851ed39bac2e6dd110b87bec.txt
-rw-r--r-- 1 www-data www-data  3823 Dec  6 14:07 log_2025-12-06_c765c0b8686ea84cf656a94f4426114e3b960f55.txt
-rw-r--r-- 1 www-data www-data  7129 Dec  8 18:41 log_2025-12-08_a489fd4b2112d06ce879ddc83f90c6b47668a61d.txt

The log filename is log_<year>-<month>-<day>_<random string>.txt. With the random string, the log filepath can't be discovered by a direct HTTP access.

In your PHP code:

global $logger; // required only in functions

$logger->info('['.__FUNCTION__.'] conf is not writeable, abort');
$logger->debug('[admin/intro::'.__LINE__.'] recent activity calculated in '.get_elapsed_time($start_time, get_moment()));