Composer errors after PHP-FPM downgrade - php

Question

I recently downgraded my PHP-FPM version from 7 to 5.6 by purging nginx and php-fpm and removing the php 7 fpm folder. It went well as my php version is now 5.6.
$ php -v
PHP 5.6.30 (cli) (built: Dec 13 2017 00:40:36)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies
My problem is whenever I do a composer install on my project, I get the following errors:
Warning: The lock file is not up to date with the latest changes in composer.json. You may be getting outdated dependencies. Run update to update them.
Your requirements could not be resolved to an installable set of packages.
Problem 1
- Installation request for behat/behat 3.2.x-dev -> satisfiable by behat/behat[3.2.x-dev].
- behat/behat 3.2.x-dev requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
Problem 2
- Installation request for behat/behat dev-master -> satisfiable by behat/behat[dev-master].
- behat/behat dev-master requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
Problem 3
- Installation request for instaclick/php-webdriver 1.4.5 -> satisfiable by instaclick/php-webdriver[1.4.5].
- instaclick/php-webdriver 1.4.5 requires ext-curl * -> the requested PHP extension curl is missing from your system.
Problem 4
- behat/behat 3.2.x-dev requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- behat/symfony2-extension dev-master requires behat/behat ~3.0,>=3.0.4 -> satisfiable by behat/behat[3.2.x-dev].
- Installation request for behat/symfony2-extension dev-master -> satisfiable by behat/symfony2-extension[dev-master].
To enable extensions, verify that they are enabled in those .ini files:
- /etc/php/7.1/cli/php.ini
- /etc/php/7.1/cli/conf.d/10-opcache.ini
- /etc/php/7.1/cli/conf.d/10-pdo.ini
- /etc/php/7.1/cli/conf.d/15-xml.ini
- /etc/php/7.1/cli/conf.d/20-calendar.ini
- /etc/php/7.1/cli/conf.d/20-ctype.ini
- /etc/php/7.1/cli/conf.d/20-dom.ini
- /etc/php/7.1/cli/conf.d/20-exif.ini
- /etc/php/7.1/cli/conf.d/20-fileinfo.ini
- /etc/php/7.1/cli/conf.d/20-ftp.ini
- /etc/php/7.1/cli/conf.d/20-gettext.ini
- /etc/php/7.1/cli/conf.d/20-iconv.ini
- /etc/php/7.1/cli/conf.d/20-json.ini
- /etc/php/7.1/cli/conf.d/20-phar.ini
- /etc/php/7.1/cli/conf.d/20-posix.ini
- /etc/php/7.1/cli/conf.d/20-readline.ini
- /etc/php/7.1/cli/conf.d/20-shmop.ini
- /etc/php/7.1/cli/conf.d/20-simplexml.ini
- /etc/php/7.1/cli/conf.d/20-sockets.ini
- /etc/php/7.1/cli/conf.d/20-sysvmsg.ini
- /etc/php/7.1/cli/conf.d/20-sysvsem.ini
- /etc/php/7.1/cli/conf.d/20-sysvshm.ini
- /etc/php/7.1/cli/conf.d/20-tokenizer.ini
- /etc/php/7.1/cli/conf.d/20-wddx.ini
- /etc/php/7.1/cli/conf.d/20-xmlreader.ini
- /etc/php/7.1/cli/conf.d/20-xmlwriter.ini
- /etc/php/7.1/cli/conf.d/20-xsl.ini
As you can see the errors still reference PHP 7 which is really odd considering my PHP version is now 5.6.
Any ideas what might be wrong? Thank you.

Answer 1

move (or remove) your lock file to a tempory name and do a composer update again . This will get rid of old data contained in your lock file and only use the most up to date data from your composer.json
mv composer.lock{,.bak} #move to composer.lock.bak
composer install #installs the project dependencies from the composer.lock file if present, or falls back on the composer.json
As an unrelated note, to make sure you are using the correct php version for your web server, create a web page in your website's root folder containing only :
<?php phpinfo();
as php -v only gives the version of the command line version.