It appears that there is a bug in the CentOS TeTeX builds somewhere that allows texvc to work from the command-line, but causes it to fail mysteriously from a PHP script with an error such as

PNG conversion failed; 
check for correct installation of 
latex, dvips, gs, and convert

It turns out that the error is due to a missing latex.fmt file. To remedy this try

fmtutil-sys --all

The solution is found here.