[root@server ~]# yum install php-mcrypt
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * addons: centos.mirror.iweb.ca
 * atomic: www6.atomicorp.com
 * base: centos.mirror.iweb.ca
 * extras: centos.mirror.iweb.ca
 * updates: centos.mirror.iweb.ca
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package php-mcrypt.i386 0:5.2.17-1.el5.art set to be updated
--> Processing Dependency: libmcrypt.so.4 for package: php-mcrypt
---> Package php-mcrypt.x86_64 0:5.2.17-1.el5.art set to be updated
--> Finished Dependency Resolution
php-mcrypt-5.2.17-1.el5.art.i386 from atomic has depsolving problems
  --> Missing Dependency: libmcrypt.so.4 is needed by package php-mcrypt-5.2.17-1.el5.art.i386 (atomic)
Error: Missing Dependency: libmcrypt.so.4 is needed by package php-mcrypt-5.2.17-1.el5.art.i386 (atomic)
 You could try using --skip-broken to work around the problem
 You could try running: package-cleanup --problems
                        package-cleanup --dupes
                        rpm -Va --nofiles --nodigest

[root@server ~]# yum info libmcrypt
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * addons: centos.mirror.iweb.ca
 * atomic: www6.atomicorp.com
 * base: centos.mirror.iweb.ca
 * extras: centos.mirror.iweb.ca
 * updates: centos.mirror.iweb.ca
Installed Packages
Name       : libmcrypt
Arch       : x86_64
Version    : 2.5.7
Release    : 5.el5.art
Size       : 591 k
Repo       : installed
Summary    : Encryption algorithms library
URL        : http://mcrypt.sourceforge.net/
License    : LGPL
Description: Libmcrypt is a thread-safe library providing a uniform interface
           : to access several block and stream encryption algorithms.
[...]

[root@server ~]# yum install php-mcrypt --skip-broken
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * addons: centos.mirror.iweb.ca
 * atomic: www6.atomicorp.com
 * base: centos.mirror.iweb.ca
 * extras: centos.mirror.iweb.ca
 * updates: centos.mirror.iweb.ca
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package php-mcrypt.i386 0:5.2.17-1.el5.art set to be updated
--> Processing Dependency: libmcrypt.so.4 for package: php-mcrypt
---> Package php-mcrypt.x86_64 0:5.2.17-1.el5.art set to be updated
--> Finished Dependency Resolution
php-mcrypt-5.2.17-1.el5.art.i386 from atomic has depsolving problems
  --> Missing Dependency: libmcrypt.so.4 is needed by package php-mcrypt-5.2.17-1.el5.art.i386 (atomic)

Packages skipped because of dependency problems:
    php-mcrypt-5.2.17-1.el5.art.i386 from atomic
    php-mcrypt-5.2.17-1.el5.art.x86_64 from atomic

yum install php-mcrypt.x86_64 --exclude=php*5.3*