If your mysql could not start while starting the xampp
 

local@host# /opt/lampp/lampp start
Starting XAMPP for Linux 1.8.2-3...
XAMPP: Starting Apache...ok.
XAMPP: Starting MySQL…
XAMPP: Couldn’t start MySQL!
XAMPP: Starting ProFTPD...ok.
XAMPP for Linux started.

We have two way solutions to fix it, one is check the Files / Directory permission  and another one is reinstall the xampp.

The First solution is try to change the files/directory permission (chmod)  and user access permission (chown)
 
local@host# chown -R nobody:root /opt/lampp
local@host# chmod 755 /opt/lampp
local@host# chmod 777 /tmp
local@host# /opt/lampp/lampp restart

after changing you have to restart the xampp services.

Incase If it does not solve this issue try  second solution, before removed / Uninstall the lampp stop the xampp and take a backup using mv or cp command.
 
local@host# mv  /opt/lampp /opt/lampp_backup

Trying to reinstall the xampp under /opt directory.
Download the xampp latest version from url below url whether the 32-bit or 64-bit version

http://sourceforge.net/projects/xampp/files/XAMPP%20Linux/1.8.3/
Have to change file permission for execute and run the installer
 
chmod 755 xampp-linux-*-installer.run
sudo ./xampp-linux-*-installer.run

The Xampp is now installed below /opt/lampp directory and can start with below command,
 
root@local:~# /opt/lampp/lampp start
Starting XAMPP for Linux 1.8.2-3...
XAMPP: Starting Apache...ok.
XAMPP: Starting MySQL...ok.
XAMPP: Starting ProFTPD...ok.

You can enjoy with Xampp!!!