PHP Warning: PHP Startup: Unable to load dynamic library 'mbstring' (tried: /usr/lib/php/20170718/mbstring (/usr/lib/php/20170718/mbstring: cannot open shared object file: No such file or directory), /usr/lib/php/20170718/mbstring.so (/usr/lib/php/20170718/mbstring.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
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Installing dependencies (including require-dev) from lock file
Your requirements could not be resolved to an installable set of packages.

Problem 1

- Installation request for erusev/parsedown 1.7.1 -> satisfiable by erusev/parsedown[1.7.1].
- erusev/parsedown 1.7.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

Problem 2

- Installation request for laravel/framework v5.6.29 -> satisfiable by laravel/framework[v5.6.29].
- laravel/framework v5.6.29 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

Problem 3

- Installation request for phpoffice/phpspreadsheet 1.3.1 -> satisfiable by phpoffice/phpspreadsheet[1.3.1].
- phpoffice/phpspreadsheet 1.3.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.

Problem 4

- Installation request for phar-io/manifest 1.0.3 -> satisfiable by phar-io/manifest[1.0.3].
- phar-io/manifest 1.0.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.

Problem 5

- Installation request for phpunit/php-code-coverage 6.0.7 -> satisfiable by phpunit/php-code-coverage[6.0.7].
- phpunit/php-code-coverage 6.0.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.

Problem 6

- Installation request for phpunit/phpunit 7.3.0 -> satisfiable by phpunit/phpunit[7.3.0].
- phpunit/phpunit 7.3.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.

Problem 7

- Installation request for theseer/tokenizer 1.1.0 -> satisfiable by theseer/tokenizer[1.1.0].
- theseer/tokenizer 1.1.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.

Problem 8

- laravel/framework v5.6.29 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- maatwebsite/excel 3.0.7 requires illuminate/support ^5.5 -> satisfiable by laravel/framework[v5.6.29].
- Installation request for maatwebsite/excel 3.0.7 -> satisfiable by maatwebsite/excel[3.0.7].

To enable extensions, verify that they are enabled in those .ini files:

- /etc/php/7.2/cli/php.ini
- /etc/php/7.2/cli/conf.d/10-opcache.ini
- /etc/php/7.2/cli/conf.d/10-pdo.ini
- /etc/php/7.2/cli/conf.d/20-calendar.ini
- /etc/php/7.2/cli/conf.d/20-ctype.ini
- /etc/php/7.2/cli/conf.d/20-exif.ini
- /etc/php/7.2/cli/conf.d/20-fileinfo.ini
- /etc/php/7.2/cli/conf.d/20-ftp.ini
- /etc/php/7.2/cli/conf.d/20-gettext.ini
- /etc/php/7.2/cli/conf.d/20-iconv.ini
- /etc/php/7.2/cli/conf.d/20-json.ini
- /etc/php/7.2/cli/conf.d/20-phar.ini
- /etc/php/7.2/cli/conf.d/20-posix.ini
- /etc/php/7.2/cli/conf.d/20-readline.ini
- /etc/php/7.2/cli/conf.d/20-shmop.ini
- /etc/php/7.2/cli/conf.d/20-sockets.ini
- /etc/php/7.2/cli/conf.d/20-sysvmsg.ini
- /etc/php/7.2/cli/conf.d/20-sysvsem.ini
- /etc/php/7.2/cli/conf.d/20-sysvshm.ini
- /etc/php/7.2/cli/conf.d/20-tokenizer.ini

You can also run php --ini inside terminal to see which files are used by PHP in CLI mode.