This script provides a form for users to register for groups with size limitation. If a user signs up for a group, its size will be incremented. Once the maximum capacity is reached, it can no longer be joined.
The form comes with frontend and backend validation and can be configured via config.inc.php, see comments. A dedicated database is not required, all data is handled via CSV.
Make sure that your server can access the .php and .htaccess files, and can write to the .csv files. Use e.g. chmod a+w groups.csv users.csv to set permissions.
Make sure that the group IDs (numbers in the first column in your groups.csv) are unique.
PHP doesn't support simple file line based editing. The script was used for 300 students over the course of one day, no technical problems were found. If you expect several submits within fractions of a second, you should probably use a real database.
If mod_ssl is loaded on your server, a http:// connection is redirected to its https:// variant via mod_rewrite. If your server does not support secure connections anyhow, you can remove the offending part in the .htaccess. This is not recommended, as the user input will be transmitted in plain text.
To prevent spam or fuzzy attacks, a maximal number of chars should be used for all input field. PHP's build-in CSV encode/decode functions are used to handle special chars, especially chars used in the CSV "syntax" (e.g. ;) will be escaped automatically.
This from uses public content delivery networks providing jQuery (and plugins) as well as the bootstrap CSS (and assets), thus the number of files can be kept minimal.
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