The most commonly used extensions are available in our Centos and Ubuntu repositories.
To install an extension in Centos, follow these steps:
$ yum install php53-soap
$ apt-get install php53-soap
Replace php53 with php54 for PHP 5.4
Other extensions may be installed with PECL.
pecl download memcache
tar -xzf memcache-X.X.X.tgz
make && make install
Then you need to enable the extension by adding the configuration to php.ini which usually is located in /usr/local/lib/php.ini
After this, restart Apache either in the control panel or via command line.