You will need to change this setting inside of php.ini
This setting can not be changed inside of your php script.
Here is a brief outline of what you will need to do (on a *NIX host):
1) find php.ini (!possibly called php5.ini)
you can use
user@host> php -i | fgrep '.ini'
or you can make a webpage and put "<?php phpinfo(); ?>" then open it in a browser, its near the top
2) copy php.ini to your php scripts directory
you can use
user@host> cp /path/to/php.ini ~webdir/upload/php.ini
or you can use an FTP client
3) open php.ini in your script's directory (with a text editor)
upload_max_filesize = xxxM
is the directive you want to edi
where xxx is an integer and M means megabyte
4) save it/done
I find that people have issues around 80MB, at which point you will need to learn more about how uploads work (I don't have timem but it's not exactly clear cut from what I can tell) or force uploaders to compress / split files.
You may also want to consider changing.
memory_limit = xxxM
post_max_size = xxxM
file_uploads = On | Off
If you intend to store the files in mysql as a blob, you will likely have to change the max packet size for mysql also.