Problema na instalação do compositor laravel em 12.04

Question

Your requirements could not be resolved to an installable set of packages.  

Problem 1    
- laravel/framework v5.3.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.26 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.25 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.24 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.23 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.21 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.20 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.19 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.18 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.17 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.16 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.15 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.14 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.13 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.12 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.11 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- laravel/framework v5.3.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.  
- Installation request for laravel/framework 5.3.* -> satisfiable by laravel/framework[v5.3.0, v5.3.1, v5.3.10, v5.3.11, v5.3.12,  
v5.3.13, v5.3.14, v5.3.15, v5.3.16, v5.3.17, v5.3.18, v5.3.19, v5.3.2,    
v5.3.20, v5.3.21, v5.3.22, v5.3.23, v5.3.24, v5.3.25, v5.3.26, v5.3.3,    
v5.3.4, v5.3.5, v5.3.6, v5.3.7, v5.3.8, v5.3.9].    

To enable extensions, verify that they are enabled in those .ini files:  
- /etc/php/5.6/cli/php.ini  
- /etc/php/5.6/cli/conf.d/10-opcache.ini  
- /etc/php/5.6/cli/conf.d/10-pdo.ini  
- /etc/php/5.6/cli/conf.d/20-calendar.ini  
- /etc/php/5.6/cli/conf.d/20-ctype.ini  
- /etc/php/5.6/cli/conf.d/20-exif.ini  
- /etc/php/5.6/cli/conf.d/20-fileinfo.ini  
- /etc/php/5.6/cli/conf.d/20-ftp.ini  
- /etc/php/5.6/cli/conf.d/20-gd.ini  
- /etc/php/5.6/cli/conf.d/20-gettext.ini  
- /etc/php/5.6/cli/conf.d/20-iconv.ini  
- /etc/php/5.6/cli/conf.d/20-json.ini  
- /etc/php/5.6/cli/conf.d/20-mcrypt.ini  
- /etc/php/5.6/cli/conf.d/20-phar.ini  
- /etc/php/5.6/cli/conf.d/20-posix.ini  
- /etc/php/5.6/cli/conf.d/20-readline.ini  
- /etc/php/5.6/cli/conf.d/20-shmop.ini  
- /etc/php/5.6/cli/conf.d/20-sockets.ini  
- /etc/php/5.6/cli/conf.d/20-sysvmsg.ini  
- /etc/php/5.6/cli/conf.d/20-sysvsem.ini  
- /etc/php/5.6/cli/conf.d/20-sysvshm.ini  
- /etc/php/5.6/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.  

Quando tentar executar este comando

composer install

    

Answer 1

Você já tentou alguma das seguintes opções para instalar a extensão de mbstring ausente:

sudo apt-get install php-mbstring

sudo apt-get install php5-mbstring

sudo apt-get install php5.6-mbstring