PHP Warning: PHP Startup: Unable to load dynamic library 'mcrypt.so'

I faced similar issue when I installed Php7.2 on Ubuntu 18. Though I had installed mcrypt using PECL still I get the error mentioned in the question.

I did following to fix it

sudo apt-get install php-pear php7.2-dev

then uninstalled

pecl uninstall mcrypt

Now reinstall mcrypt

sudo apt-get -y install gcc make autoconf libc-dev pkg-configsudo apt-get -y install libmcrypt-devsudo pecl install mcrypt-1.0.1

When you are shown the prompt

libmcrypt prefix? [autodetect] :

Press [Enter] to autodetect.

After success installing mcrypt using pecl, you should add mcrypt.so extension to php.ini.

The output will look like this:

...Build process completed successfullyInstalling '/usr/lib/php/20170718/mcrypt.so'    ---->   this is our path to mcrypt extension libinstall ok: channel://pecl.php.net/mcrypt-1.0.1configuration option "php_ini" is not set to php.ini locationYou should add "extension=mcrypt.so" to php.ini

Now restart Apache

sudo service apache2 restart

Grab installing path and add to cli and apache2 php.ini configuration.

sudo bash -c "echo extension=/usr/lib/php/20170718/mcrypt.so > /etc/php/7.2/cli/conf.d/mcrypt.ini"sudo bash -c "echo extension=/usr/lib/php/20170718/mcrypt.so > /etc/php/7.2/apache2/conf.d/mcrypt.ini"

First, open up a terminal window and install the necessary dependencies with the commands:

sudo apt-get -y install gcc make autoconf libc-dev pkg-configsudo apt-get -y install php7.2-devsudo apt-get -y install libmcrypt-dev

Once the dependencies have been installed, you can install mcrypt with the command:

sudo pecl install mcrypt-1.0.1

And there you go. Mcrypt is now installed. Go back to the process of installing whatever server software that depends upon this extension and you should be good to go.

For (>= PHP 7.3) you can use the following command:

sudo pecl install mcrypt-1.0.2