Description:
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You can't just upgrade php<5.3 to >=5.3 with yum update because they are 
different packages. Ok, I understand that was decided because 5.3 breaks some 
backward compatibility.

But then, you can't install 5.3 without uninstalling 5.1 either, because:

  php53-common conflicts with php-common

That's RIDICULOUS and inconsistent.

If it is impossible to upgrade 5.1/2 to 5.3, then it MUST be possible to install 
both. If they are different packages, then they must be compatible!!!

One may be unable to uninstall 5.1 because there are packages that depend on it 
(e.g. plesk).