Joomla! Documentation™

For MAMP

Steps:

• Go to MAMP application >> Preferences (button) >> PHP (tab) >> Choose 5.3.1 or greater >> choose OK.
• Go to the MAMP folder >> bin >> php >> php5.3.6 >> conf >> edit php.ini >> add "magic_quotes_gpc = Off" a few lines above "magic_quotes_sybase = Off".
• Restart MAMP's servers.

For most mainstream hosting companies running a CGI-Webinterface

Steps:

• Create a php.ini or php5.ini file with the following:

magic_quotes_gpc = Off

• Put it in your Joomla 3 root. Then change the htaccess.txt in your Joomla 3 root to .htaccess. Add the following lines to the .htaccess file (at the top), don't forget to change php.ini to php5.ini when applicable :

<IfModule mod_suphp.c>
  suPHP_ConfigPath /home/myusername/public_html/yourJ3folder
  <Files php.ini>
    order allow,deny
    deny from all
  </Files>
</IfModule>

Change "myusername" and "yourJ3folder" to your respective folders. The "/home/myusername/public_html/yourJ3folder" can be found via the Global Configuration:

In Joomla backend > System > System Information > [Directory Permissions], the folder is usually the same as the log directory (but without /logs at the end).

.htaccess for some hosts

For some hosts, add "php_flag magic_quotes_gpc off" to the .htaccess file

Yet Another Solution For Shared Hosts

• create a php.ini file at your Joomla! root.
• Add this content to the file and save

magic_quotes_gpc = Off
magic_quotes_runtime = Off
magic_quotes_sybase = Off
extension=pdo.so
extension=pdo_mysql.so

• Edit your .htaces file and add this line at the top and save the file

SetEnv PHPRC /home/youruser/public_html/php.ini

• Test if the error message goes away

source : http://stackoverflow.com/questions/517008/how-to-turn-off-magic-quotes-on-shared-hosting