Follow these steps to install APC on a RHEL 5 server.

1. Install php-devel for phpize (if not installed).

yum install php-devel

2. Install the apache devel pkgs for apxs (if needed).

yum install httpd-devel

3. Make /tmp executable.

mount -o remount,exec /tmp

4. Set the memory limit in /usr/share/pear/pearcmd.php to 16M.

vi /usr/share/pear/pearcmd.php
@ini_set('memory_limit', '16M');

5. Install apc.

pecl install apc

6. Add a /etc/php.d/apc.ini with the following line:

extension=apc.so

7. Restart apache (check configuration first for errors).

service httpd configtest (watch for errors)
service httpd restart

Remove the ini_set and set /tmp back to noexec.