ph5diff: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/lib64/mpich/bin/ph5diff - from package: hdf5-mpich
/usr/lib/mpich/bin/ph5diff - from package: hdf5-mpich
/usr/lib64/openmpi/bin/ph5diff - from package: hdf5-openmpi
/usr/lib/openmpi/bin/ph5diff - from package: hdf5-openmpi

You may install the required package with command yum install {package}

Error in other languages:
ph5diff: bevel nie gevind nie
ph5diff: comandă negăsită
ph5diff: comando não encontrado
ph5diff: commande introuvable
ph5diff: command not found
ph5diff: käsku ei ole
ph5diff: không tìm thấy lệnh
ph5diff: komanda nerasta
ph5diff: Kommando nicht gefunden.
ph5diff: kommandot finns inte
ph5diff: komut yok
ph5diff: nie znaleziono polecenia
ph5diff: níor aimsíodh an t-ordú
ph5diff: no se encontró la orden
ph5diff: no s'ha trobat l'ordre
ph5diff: opdracht niet gevonden
ph5diff: parancs nem található
ph5diff: perintah tidak ditemukan
ph5diff: príkaz nenájdený
ph5diff: příkaz nenalezen
ph5diff: команда не найдена
ph5diff: командата не е открита
ph5diff: 命令找不到
ph5diff: コマンドが見つかりません