Description:
------------
PHP7-PHP8 is not releasing "Resident Memory" back to the OS after use (specially Wordpress sites). PHP5 works as expected, example:

PHP5  : 10MB ---> 95MB ---> 10MB
PHP7+ : 10MB ---> 95MB ---> 95MB

Versions Used:
* PHP7 versions 7.2, 7.3, 7.4 and 8.0 (running as FastCGI in lighttpd)
* Linux OS: Alpine 3.10, Alpine 3.14 and Ubuntu 20.04
* Wordpress: 5.7, 5.8

I have tested it without Wordpress (using the provided test script) but the result is not conclusive (across versions), however when using Wordpress, it always happen.

We noticed after several sites (about 500 sites) were updated from PHP 5 to PHP 7 (running in LXD containers), as the memory consumption increased about 3 to 4 times, making the server performance to go down drastically (server started to swap more often). Even when the sites are not accessed for a long time, the memory is not released.

Settings: Almost all sites are running the default settings (opcache is OFF)

Memory checking tools: `htop`, `top` and `lxc info`

Modules used:

ctype
curl
date
dom
filter
gd
hash
iconv
json
libxml
mbstring
mysqli
mysqlnd
openssl
pcre
readline
Reflection
session
SPL
standard
xml
xmlreader
xmlrpc
xmlwriter
zip
zlib


Test script:
---------------
<?php
// NOTE: this code SOMETIMES produces the problem described above, 
//       and sometimes it won't.
$m = "";
for($i = 0; $i < 110000; $i++) {
    $m .= "iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii";
}
sleep(3);
echo "DONE";
?>