I can't connect to redis with php

I can’t connect to redis with php.
I built an environment with docker and tried to connect to redis from php with a php and redis container, but it failed. I get the following error.

PHP Warning:  PHP Startup: Unable to load dynamic library 'redis.so' (tried: /usr/lib/php/20190902/redis.so (/usr/lib/php/20190902/redis.so: undefined symbol: php_json_decode_ex), /usr/lib/php/20190902/redis.so.so (/usr/lib/php/20190902/redis.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
PHP Notice:  session_start(): Redis not available while creating session_id in /var/www/html/index.php on line 2
PHP Warning:  session_start(): Failed to read session data: redis (path: tcp://localhost:6379) in /var/www/html/index.php on line 2
string(0) ""
save_handler=redis
save_path=tcp://localhost:6379
session_id=

This is the php file that was executed in response to the error.

<?php
session_start();

ini_set('session.cookie-domain', 'localhost');

var_dump(session_id());
echo "save_handler=" . ini_get("session.save_handler") . "n";
echo "save_path=" . ini_get("session.save_path") . "n";
echo "session_id=" . session_id() . "n";

$_SESSION['libname'] = "PhpRedis";
?>

Part of the php.ini file.

[Session]
; Handler used to store/retrieve data.
; http://php.net/session.save-handler
session.save_handler = redis
session.save_path = "tcp://localhost:6379"

;verifying redis extension module
extension = redis.so

; default redis timeout
redis.timeout = 5

Please let me know if I am missing any information necessary to resolve this issue. Thank you in advance.

p.s.
I typed ping and it returned pong.
I can connect to redis from the php container with redis-cli. Also, here is the path to redis.so.
~/usr/lib/php/20190902/redis.so
The path to php.ini (server used is apache2).
~/etc/php/7.4/apache2/php.ini