What are some best practices for form submissions and data validation in PHP applications?

Validate All Input Data: Validate all input data, including required fields and optional fields, to ensure that no invalid data is submitted to the server. ... Use Whitelisting over Blacklisting: Use whitelisting techniques to validate input data rather than blacklisting techniques.

Right now many packages from composer which supports the best way validate form submission. If you are using Laravel framework, it has amazing validation method inside its framework. If you are using Slim framework, you can use Rakit Validation, which similar with Laravel Validation. I also have managed an application using PHP before framework was booming previously. It's use regex, filter function on built-in PHP language. It's so hard to adjust when a bug happened on this framework. Most of form request includes function to insert into unique column of a table. It's must validation by querying SQL. Many framework has supported to do it.

When using PHP for form submission CSRF token should be taken into account. This method is known as creating a special token that is put inside the form and assigned to every user’s session. At submission of the form, the server validates the token to ensure the request is genuine and not forged. This is an essential measure in ensuring that no unauthorized operation done with respect to logged-in users affects your application. Although it’s still necessary to focus on server-side validation and data sanitation, emphasis should be placed on CSRF defense, enhancing security measures against attacks that target the app along with protected user information.

Validate client side first, validate more with receiving/server side code like php/python, and then validate a final time at the database. A simple form is not simple, it takes a lot of work.

One of the interesting ways with which one can validate data within PHP is known as regular expressions or regex. It allows you to specify patterns that will be used to confirm whether the information matches what you expect, for instance, phone numbers or e-mail addresses. Regex is quite exhaustive such that you will be certain of the exactness of the information. However, it is somewhat difficult to get started with its complicated rules. However, with practice, though, RegEx becomes another formidable tool to ensure your data is perfectly scrutinized.

Always sanitize your input in frontend and backend. Another case, don't use MySQL field name for query directly from user input

Setting up a log system in your PHP app helps in managing errors and exceptions when sending form submissions to the database. Error messages in this system are not restricted to only that; it can be seen as a diary where any error issue is noted. In case of an error, the system captures it along with all the details including the nature of the error, when it occurred, and the state of your app. It may prove useful by highlighting patterns or repeated areas of concern through which you analyze issues intending to resolve them. This is very crucial in creating a robust and reliable application where it is important to catch things that are hidden.

I think the proposals they have made are good, I have never used CRSF, but I think I will put it into practice. It catches my attention that it focuses on validations, and whether they should be done in the html, but also within php, when receiving the data from the form.

BDD(Behavior-Driven Development) is a good approach to testing and fixing your PHP codes that mostly involve forms and data validation. In other words, BDD has everything regarding ensuring that your application functions as your users would want. Unlike technical tests, BDD allows one to draft tests that look more like simple stories, describing how the application should function from a user’s standpoint. Thus, you’re not only ensuring that the code is correct, but that it is understandable by users of your application. In PHP, there is a tool known as Behat used for this. You can find out the problems ahead of time as long as the forms in your app work exactly how your users want them to.

Always using the rate limit for submission form so people or attackers can't submit repeatedly to your database and make your database peak up.