Increasing Upload Limits PHP
By default php sets the max upload filesize to 2mb
To view your current php settings create the following file
Save it as info.php and upload to your website then view it in your browser.
The two important variables are
Next step is to change those settings. Look for "Configuration File (php.ini) Path " and
"Loaded Configuration File" on the phpinfo page to find your php settings are stored.
Edit that file and increase upload_max_filesize and post_max_size to the size of the largest file you wish to be allowed uploaded.
Then after your save those changes be sure to restart your webserver for the changes to take affect.
If you do not have access to the php.ini you can still override it in most cases on apache by creating a .htaccess file and place it in the root directory of your forum. If the file already exists just append these files to it.
Example .htaccess file
php_value upload_max_filesize 20M
php_value post_max_size 20M
Also if you are using Fast CGI mod_fcgi with apache you need to increase the
MaxRequestLen - variable in the fastcgi settings