Your requirements could not be resolved to an installable set of packages.
Problem 1
- Installation request for laravel/framework v5.2.39 -> satisfiable by laravel/framework[v5.2.39].
- laravel/framework v5.2.39 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
Problem 2
- Installation request for league/commonmark 0.13.4 -> satisfiable by league/commonmark[0.13.4].
- league/commonmark 0.13.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
Problem 3
- Installation request for simplesoftwareio/simple-qrcode 1.3.3 -> satisfiable by simplesoftwareio/simple-qrcode[1.3.3].
- simplesoftwareio/simple-qrcode 1.3.3 requires ext-gd * -> the requested PHP extension gd is missing from your system.
Problem 4
- laravel/framework v5.2.39 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- roumen/feed v2.10.4 requires illuminate/support ^5.0 -> satisfiable by laravel/framework[v5.2.39].
- Installation request for roumen/feed v2.10.4 -> satisfiable by roumen/feed[v2.10.4].
1$ yum search mbstring
2読み込んだプラグイン:fastestmirror, security
3Repository pgdg93 is listed more than once in the configuration
4Repository pgdg93-source is listed more than once in the configuration
5Determining fastest mirrors
6 * base: ftp.iij.ad.jp
7 * epel: mirror.dmmlabs.jp
8 * extras: ftp.iij.ad.jp
9 * remi: ftp.riken.jp
10 * remi-php70: ftp.riken.jp
11 * remi-php71: ftp.riken.jp
12 * remi-safe: ftp.riken.jp
13 * rpmforge: ftp.riken.jp
14 * updates: ftp.iij.ad.jp
15Webmin 119/119
16epel 12516/12516
17wandisco-svn 115/115
18================================================== N/S Matched: mbstring ==================================================19php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
20php54-php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
21php55-php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
22php56-php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
23php70-php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
24php71-php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
25php72-php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
26php73-php-mbstring.x86_64 : A module for PHP applications which need multi-byte string handling
2728 Name and summary matches only, use "search all"for everything.
AmazonLinuxだと↓とかでどうしょうか
https://lab.maro-log.net/post-12390/
(手順中yum list php7.1* => yum list php71*としてください)
おそらくCentOSの方でもできると思いますが、こちらでもmbstringのインストールができると思います。