phpmd: 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/phpmd - from package: php-phpmd-PHP-PMD

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

On Debian possible default full paths are:

/usr/bin/phpmd - from package: phpmd

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

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