jon
You must have changed your repositories to point at sme8/el5/CentOS sources. Why did you do that as those rpms are not applicable to sme7.6.
There will be very few updates released for sme7.6 as the underlying CentOS 4.x base is "end of life" (EOL). There may be an occasional security update if deemed critical, & clamav updates (as now just released), but no further development will take place. Eventually sme7.6 will becone insecure.
You would be best to resolve your PHP issues & move forward by upgrading to sme8.
Every problem has an answer.
As far as resetting your yum repositories to defaults, that info has been published on contribs.org, I think in the FAQ, so start reading/searching. IfI come acroos the link I will add it here.
Here it is (linked to from an entry in the FAQ)
http://wiki.contribs.org/SME_Server:Adding_Software#Restoring_Default_Yum_Repositories