Where is the mb_string.so installed by apt installed?

problem description

php wrote an interface to reset the password, and everything was normal in the local test. If you put it on the server, it would cause a 502 error. The environment is all built by docker, exactly the same, and you can"t think of riding a sister.
then log in to the server and check the log and find that there is always a warning

mbstring module I installed with the apt command, and the related functions have been used normally.
the so file was not found in the directory that opened the prompt.
but check that php-m is installed normally.
so I want to copy the .so file to the "usr/lib/php/20151012/"" directory.
ask everyone where the mb_string.so installed by apt is installed.

Php
Mar.31,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3e9af-2c421.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3e9af-2c421.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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