yum install php-mbstring

Installing:
 php-mbstring                              x86_64                              5.3.6-3.el5.remi                                remi                              2.3 M
Updating for dependencies:
 mysql                                     x86_64                              5.5.11-1.el5.remi                               remi                              7.4 M
 mysql-libs                                x86_64                              5.5.11-1.el5.remi                               remi                              1.1 M
 mysql-server                              x86_64                              5.5.11-1.el5.remi                               remi                               12 M
 php                                       x86_64                              5.3.6-3.el5.remi                                remi                              1.4 M
 php-cli                                   x86_64                              5.3.6-3.el5.remi                                remi                              2.6 M
 php-common                                x86_64                              5.3.6-3.el5.remi                                remi                              973 k
 php-devel                                 x86_64                              5.3.6-3.el5.remi                                remi                              623 k
 php-gd                                    x86_64                              5.3.6-3.el5.remi                                remi                              208 k
 php-mysql                                 x86_64                              5.3.6-3.el5.remi                                remi                              159 k
 php-pdo                                   x86_64                              5.3.6-3.el5.remi                                remi                              117 k
 php-xml                                   x86_64                              5.3.6-3.el5.remi                                remi                              221 k

Transaction Summary
=======================================================================================================================================================================
Install      1 Package(s)
Update      11 Package(s)
Remove       0 Package(s)

yum info installed php

yum install php-mbstring-5.3.5-1.el5.remi