PHP powers a huge share of the web — most content management systems, countless custom backends, and a large fraction of e-commerce sites run on it. That popularity is exactly why PHP applications are such a frequent target: attackers write one exploit and can try it against millions of sites built on the same frameworks and the same common mistakes. None of this is a flaw unique to PHP as a language. The vulnerabilities covered in this section — injection, cross-site scripting, cross-site request forgery, weak authentication — show up in every server-side language. PHP just happens to be where a beginner is most likely to first meet them, because so many beginners write their first dynamic, database-backed website in it.

This section is a short, practical security curriculum built around the categories of attack a working PHP developer is most likely to encounter. It borrows its shape from the kind of category list you will see in the OWASP Top Ten, not because you need to memorize an official standard, but because those categories represent decades of real incident reports condensed into a short list.

Almost every vulnerability in this section reduces to the same root cause: a value that came from outside the application — a form field, a query string parameter, an HTTP header, a cookie, an uploaded file, even data pulled from a third-party API — was treated as if it were safe, trusted data written by the developer. It was not. Anything that originates outside your own code should be treated as hostile until it has been validated, and it should be escaped or parameterized for whatever context it is about to enter — a SQL query, an HTML page, a shell command, a file path — because each of those contexts has its own rules about what makes a value dangerous.

In PHP terms, this means every value pulled from $_GET, $_POST, $_COOKIE, $_SERVER, $_FILES, or even $_SESSION (if it was ever populated from earlier user input) deserves scrutiny before it is used. It does not matter that a hidden form field "should" only ever contain a number, or that a dropdown "should" only ever contain one of five values — an attacker does not have to use your form at all. They can send any HTTP request they like directly to your server, with any field set to any value.

A single well-placed fix — say, switching to prepared statements — eliminates SQL injection on its own. But secure applications rarely rely on exactly one safeguard for each threat. Defense in depth means stacking multiple, independent layers of protection so that if one layer has a bug, gets misconfigured, or is simply forgotten on one code path, another layer still catches the problem.