I added a compat library for that condition the other day. See if it installs now

yum upgrade php
Setting up Upgrade Process
Setting up repositories
atomic 100% |=========================| 1.9 kB 00:00
Reading repository metadata in from local files
primary.xml.gz 100% |=========================| 247 kB 00:00
atomic : ################################################## 700/700
Resolving Dependencies
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for php to pack into transaction set.
php-5.3.8-2.el4.art.i386. 100% |=========================| 21 kB 00:00
---> Package php.i386 0:5.3.8-2.el4.art set to be updated
--> Running transaction check
--> Processing Dependency: php = 4.3.9-3.31 for package: php-gd
--> Processing Dependency: php = 4.3.9-3.31 for package: php-domxml
--> Processing Dependency: php = 4.3.9-3.31 for package: php-imap
--> Processing Dependency: php = 4.3.9-3.31 for package: php-pgsql
--> Processing Dependency: php = 4.3.9-3.31 for package: php-mysql
--> Processing Dependency: /usr/bin/php for package: psa-horde
--> Processing Dependency: php-common = 5.3.8-2.el4.art for package: php
--> Processing Dependency: php = 4.3.9-3.31 for package: php-pear
--> Processing Dependency: php-cli = 5.3.8-2.el4.art for package: php
--> Processing Dependency: php = 4.3.9-3.31 for package: php-mbstring
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for php-common to pack into transaction set.
php-common-5.3.8-2.el4.ar 100% |=========================| 27 kB 00:00
---> Package php-common.i386 0:5.3.8-2.el4.art set to be updated
---> Downloading header for php-mysql to pack into transaction set.
php-mysql-5.3.8-2.el4.art 100% |=========================| 21 kB 00:00
---> Package php-mysql.i386 0:5.3.8-2.el4.art set to be updated
---> Downloading header for php-cli to pack into transaction set.
php-cli-5.3.8-2.el4.art.i 100% |=========================| 21 kB 00:00
---> Package php-cli.i386 0:5.3.8-2.el4.art set to be updated
---> Downloading header for php-gd to pack into transaction set.
php-gd-5.3.8-2.el4.art.i3 100% |=========================| 20 kB 00:00
---> Package php-gd.i386 0:5.3.8-2.el4.art set to be updated
---> Downloading header for php-imap to pack into transaction set.
php-imap-5.3.8-2.el4.art. 100% |=========================| 20 kB 00:00
---> Package php-imap.i386 0:5.3.8-2.el4.art set to be updated
---> Downloading header for php-mbstring to pack into transaction set.
php-mbstring-5.3.8-2.el4. 100% |=========================| 20 kB 00:00
---> Package php-mbstring.i386 0:5.3.8-2.el4.art set to be updated
---> Downloading header for php-xml to pack into transaction set.
php-xml-5.2.17-1.el4.art. 100% |=========================| 17 kB 00:00
---> Package php-xml.i386 0:5.2.17-1.el4.art set to be updated
---> Downloading header for php-pgsql to pack into transaction set.
php-pgsql-5.3.8-2.el4.art 100% |=========================| 21 kB 00:00
---> Package php-pgsql.i386 0:5.3.8-2.el4.art set to be updated
---> Downloading header for php-pear to pack into transaction set.
php-pear-1.7.2-2.el4.art. 100% |=========================| 28 kB 00:00
---> Package php-pear.noarch 1:1.7.2-2.el4.art set to be updated
--> Running transaction check
--> Processing Dependency: php-pdo for package: php-pgsql
--> Processing Dependency: php-pdo for package: php-mysql
--> Processing Dependency: libedit.so.0 for package: php-cli
--> Processing Dependency: libmysqlclient.so.16(libmysqlclient_16) for package: php-mysql
--> Processing Dependency: php-common = 5.2.17-1.el4.art for package: php-xml
--> Processing Dependency: libmysqlclient.so.16 for package: php-mysql
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for mysql-libs to pack into transaction set.
mysql-libs-5.1.59-2.el4.a 100% |=========================| 41 kB 00:00
---> Package mysql-libs.i386 0:5.1.59-2.el4.art set to be updated
---> Downloading header for libedit to pack into transaction set.
libedit-3.0-2.20090923cvs 100% |=========================| 3.8 kB 00:00
---> Package libedit.i386 0:3.0-2.20090923cvs.el4.art set to be updated
---> Downloading header for php-pdo to pack into transaction set.
php-pdo-5.3.8-2.el4.art.i 100% |=========================| 20 kB 00:00
---> Package php-pdo.i386 0:5.3.8-2.el4.art set to be updated
--> Running transaction check
--> Processing Dependency: mysql = 5.1.59-2.el4.art for package: mysql-libs
--> Processing Dependency: php-common = 5.2.17-1.el4.art for package: php-xml
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for mysql to pack into transaction set.
mysql-5.1.59-2.el4.art.i3 100% |=========================| 37 kB 00:00
---> Package mysql.i386 0:5.1.59-2.el4.art set to be updated
--> Running transaction check
--> Processing Dependency: libmysqlclient.so.14 for package: perl-DBD-MySQL
--> Processing Dependency: libmysqlclient.so.14 for package: psa-mail-qc-driver
--> Processing Dependency: libmysqlclient_r.so.14 for package: mysql-server
--> Processing Dependency: mysql = 4.1.22-2.el4_8.4 for package: mysql-server
--> Processing Dependency: libmysqlclient.so.14 for package: psa-backup-manager
--> Processing Dependency: libmysqlclient_r.so.14(libmysqlclient_14) for package: mysql-server
--> Processing Dependency: /usr/lib/mysql/libmysqlclient.so.14 for package: sw-engine
Importing Additional filelist information for dependency resolution
filelists.xml.gz 100% |=========================| 485 kB 00:00
atomic : ################################################## 700/700
--> Processing Dependency: libmysqlclient.so.14 for package: psa-mail-driver-common
--> Processing Dependency: libmysqlclient.so.14 for package: mysql-server
--> Processing Dependency: php-common = 5.2.17-1.el4.art for package: php-xml
--> Processing Dependency: libmysqlclient.so.14 for package: psa
--> Processing Dependency: libmysqlclient.so.14(libmysqlclient_14) for package: mysql-server
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for mysqlclient14 to pack into transaction set.
mysqlclient14-4.1.22-2.el 100% |=========================| 4.9 kB 00:00
---> Package mysqlclient14.i386 0:4.1.22-2.el4.art set to be updated
---> Downloading header for mysql-server to pack into transaction set.
mysql-server-5.5.18-1.el4 100% |=========================| 38 kB 00:00
---> Package mysql-server.i386 0:5.5.18-1.el4.art set to be updated
--> Running transaction check
--> Processing Dependency: mysql = 5.5.18-1.el4.art for package: mysql-server
--> Processing Dependency: libaio.so.1(LIBAIO_0.4) for package: mysql-server
--> Processing Dependency: mysql-libs = 5.5.18-1.el4.art for package: mysql-server
--> Processing Dependency: libaio.so.1 for package: mysql-server
--> Processing Dependency: libaio.so.1(LIBAIO_0.1) for package: mysql-server
--> Processing Dependency: php-common = 5.2.17-1.el4.art for package: php-xml
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for mysql to pack into transaction set.
mysql-5.5.18-1.el4.art.i3 100% |=========================| 29 kB 00:00
warning: package mysql = 5.1.59-2.el4.art was already added, replacing with mysql <= 5.5.18-1.el4.art
---> Package mysql.i386 0:5.5.18-1.el4.art set to be updated
---> Downloading header for mysql-libs to pack into transaction set.
mysql-libs-5.5.18-1.el4.a 100% |=========================| 32 kB 00:00
warning: package mysql-libs = 5.1.59-2.el4.art was already added, replacing with mysql-libs <= 5.5.18-1.el4.art
---> Package mysql-libs.i386 0:5.5.18-1.el4.art set to be updated
--> Running transaction check
--> Processing Dependency: libmysqlclient.so.16(libmysqlclient_16) for package: php-mysql
--> Processing Dependency: libaio.so.1(LIBAIO_0.4) for package: mysql-server
--> Processing Dependency: libaio.so.1 for package: mysql-server
--> Processing Dependency: libaio.so.1(LIBAIO_0.1) for package: mysql-server
--> Processing Dependency: php-common = 5.2.17-1.el4.art for package: php-xml
--> Processing Dependency: libmysqlclient.so.16 for package: php-mysql
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Package mysql-libs.i386 0:5.1.59-2.el4.art set to be updated
---> Package mysql.i386 0:5.1.59-2.el4.art set to be updated
---> Downloading header for mysqlclient16 to pack into transaction set.
mysqlclient16-5.1.59-2.el 100% |=========================| 3.7 kB 00:00
---> Package mysqlclient16.i386 0:5.1.59-2.el4.art set to be updated
--> Running transaction check
--> Processing Dependency: libaio.so.1(LIBAIO_0.1) for package: mysql-server
--> Processing Dependency: php-common = 5.2.17-1.el4.art for package: php-xml
--> Processing Dependency: libaio.so.1 for package: mysql-server
--> Processing Dependency: libaio.so.1(LIBAIO_0.4) for package: mysql-server
--> Finished Dependency Resolution
Error: Missing Dependency: php-common = 5.2.17-1.el4.art is needed by package php-xml
Error: Missing Dependency: libaio.so.1(LIBAIO_0.4) is needed by package mysql-server
Error: Missing Dependency: libaio.so.1 is needed by package mysql-server
Error: Missing Dependency: libaio.so.1(LIBAIO_0.1) is needed by package mysql-server