Here is how you can upgrade your version php 5.1x on your plesk install to php 5.2x

wget -q -O - http://www.ucvhost.com/installers/atomic.sh |sh
yum update php
yum install php-soap
yum install php-json
/usr/local/psa/admin/sbin/websrvmng -a
service psa stopall
service psa start

– vishwa at work enjoy !!!