mcrypt: 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/bin/mcrypt - from package: mcrypt

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

On Debian possible default full paths are:

/usr/bin/mcrypt - from package: mcrypt

You may install the required package with command apt-get install {package}

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