You need to install epel repository to get full php features on centos/RH. Phalcon will be there too.
$ cat /etc/*-release
CentOS release 6.6 (Final)
LSB_VERSION=base-4.0-amd64:base-4.0-noarch:core-4.0-amd64:core-4.0-noarch
CentOS release 6.6 (Final)
CentOS release 6.6 (Final)
$ yum repolist
Loaded plugins: presto, priorities, security
2444 packages excluded due to repository priority protections
repo id repo name status
base CentOS-6 - Base 6,518
elrepo ELRepo.org Community Enterprise Linux Rep 324+2
epel Extra Packages for Enterprise Linux 6 - x 10,440+786
extras CentOS-6 - Extras 35
nginx-release nginx repo 69
remi Les RPM de remi pour Enterpise Linux 6 - 2,342
rpmforge-rhel5 RPMForge RHEL5 packages 3,166+1,552
updates CentOS-6 - Updates 388
repolist: 24,671
$ yum install php-phalcon
Loaded plugins: presto, priorities, security
Setting up Install Process
2444 packages excluded due to repository priority protections
Package php-phalcon-1.3.4-1.el6.remi.5.4.x86_64 already installed and latest version
Nothing to do