ph.x: command not found

Have You tried accessing the binary with the full path?

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/bin/ph.x - from package: quantum-espresso

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

On Debian possible default full paths are:

/usr/bin/ph.x - from package: quantum-espresso

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