Although I have very little experience with CentOS, I do seem to recall seeing some flavor of linux that didn't have sendmail configured with a queue runner by default. I would be curious to see if your old messages are removed after running 'sendmail -q'. If that's the case, then I think you just need to configure your queue runner to run periodically.
You have a bad installation somehow.
It is conventional on Unix that a program exists in /usr/sbin/sendmail$, which acts as an interface to the local MTA. This MTA is rarely sendmail nowadays, but other MTA's have a compatible program which is installed here.
sendmail.h would be part of the source code for sendmail, and sendmail.0 and sendmail.8 would be the man pages, so it looks like you've somehow got the source code for the right sendmail in /usr/sbin
You need to clean this up, and get a proper sendmail program into /usr/sbin/sendmail to fix php. Since you've not told us the distribution, nor which MTA you're actually using, no-one else can help you.
$ = Actually one or more of /usr/lib/sendmail and /usr/bin/sendmail as well as /usr/sbin/sendmail, programs will either search these paths, or have one or more hardcoded into them, or ask at installation time. It looks like your program is using /usr/sbin/sendmail. Regardless of which, the installation for the MTA will do it.
Best Answer
From the bat book: