BlueLake
Considering your name, it's probably easier for you to walk on water, but for others, it's probably easier to nominate a suitable repo (repository) that has the required package.
That package is not in the standard repos that SME server uses by default, that's why the command you used is not finding & installing it.
I do not know where php5-pgsql is, or if it exists, but you might try epel, or dag or atrpms or webtatic repos.
If you can tell us where that package is, then we can tell you how to install it on SME server
Then you install it using a command like
yum install --enablerepo=epel php5-pgsql
(assuming you have first configured epel repo on your sme server & second that the package is in that repo, replace repo name as appropriate).
See the FAQ, link at top of forums, Section 1 Installation, for everything you need to know about repos, how to add & enable them etc.
particularly
http://wiki.contribs.org/SME_Server:Documentation:FAQ:Section01#Other_popular_repositoriesand
http://wiki.contribs.org/Category:Yum_Repository Be aware you may have compatibility & dependency issues installing additional packages like that, so proceed carefully.
I checked & it's not in dag, epel, webtatic or atrpms (for 32 bit system, there might be a 64 bit pkg if your system is 64 bit, so try), otherwise you will have to search around for it.
If you configure all those repos on SME, then run the command suggested (modified to suit each repo), & it will show you if that pkg is available or not, you can cancel the install also.
I googled & found rpm.pbone.net & rpmfind.net has a 64 bit pkg, so download it to a empty folder on your sme server & do a local yum install
wget
http://websiteURLpath/filename.rpmyum localinstall fullpackagename.rpm
signal-event post-upgrade
signal-event reboot