indexer.php: 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/share/dokuwiki/bin/indexer.php - from package: dokuwiki

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

On Debian possible default full paths are:

/usr/share/dokuwiki/bin/indexer.php - from package: dokuwiki

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

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