Try increasing the following values in php.ini, for example:
memory_limit = 32M upload_max_filesize = 24M post_max_size = 32M
After making these changes, you may need to restart Apache.
/etc/init.d/httpd restart OR sudo service apache2 restart
If the upload_max_filesize is larger than post_max_size, you must increase post_max_size so that it is bigger than upload_max_size.
If the value of post_max_size is larger than memory_limit, you must increase memory_limit so that it is larger than post_max_size.