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You can increase the maximum upload file size for only one application by creating a php.ini file in the directory where the file upload PHP script is. Add the following lines:
; Maximum allowed size for uploaded files.
upload_max_filesize = 40M
; Must be greater than or equal to upload_max_filesize
post_max_size = 40M
It can also be changed system wide by editing the file /etc/php5/apache2/php.ini which is the global php.ini file. Modify the two lines listed above and restart apache.