The first time I formally pentested one of my own billing applications — not just "poke at it and see what breaks" but a structured, documented engagement — I found three issues I genuinely hadn't anticipated, including a second-order SQL injection in the invoice search function that had been sitting there for eight months. That experience turned me into a believer in having a real methodology. If you're a PHP developer who wants to start pentesting your own applications or work toward professional security testing, this framework is where I'd tell you to start. Everything here stays within legal, responsible boundaries — which means you only test systems you own or have explicit written permission to test.

Scope Definition and Rules of Engagement

Before touching any tool, you need a written scope document. For testing your own app this is a memo to yourself; for a client engagement it's a formal rules of engagement (ROE) document. The ROE answers: what hosts and IP ranges are in scope, what testing activities are authorized (passive recon only? active scanning? exploitation?), what time windows are allowed (avoid business hours for active scans on production), and what the escalation path is if you find a critical vulnerability that's being actively exploited.

For a typical PHP web app pentest I scope it as: the target domain and all subdomains, web application layer only (no network infrastructure unless specifically included), active scanning allowed during agreed maintenance windows, and exploitation limited to proof-of-concept with no data exfiltration. Write this down. Undocumented scope creep is how legitimate testers get into legal trouble.

Reconnaissance: Passive Before Active

Passive recon means gathering information without sending packets to the target. This is always legal and always first. I use a combination of:

  • Google dorks — site:target.com filetype:php to find exposed PHP files, site:target.com inurl:admin to find admin panels indexed by Google
  • Wayback Machine — archived versions of the site often reveal old endpoints, old parameters, and previous technology stack choices
  • Certificate Transparency logs via crt.sh — shows all SSL certificates issued for a domain, revealing subdomains the target may not want public
  • Shodan and Censys for Internet-exposed services on the target IP range
  • LinkedIn and job postings — a job post reading "must know PHP 8.1, Laravel, MySQL 8.0, and Plesk" tells me a lot about the stack before I've sent a single packet

Active recon is sending traffic directly to the target. This starts with a port scan using Nmap to understand what services are exposed, followed by HTTP fingerprinting to identify the server version, PHP version, and any visible framework signatures:

nmap -sV -sC -p 80,443,8080,8443 target.com
curl -I https://target.com

The Server: and X-Powered-By: response headers often reveal Apache/Nginx versions and PHP versions. Modern hardened configurations suppress these, but you'd be surprised how many billing apps still broadcast X-Powered-By: PHP/7.4.3 — which tells an attacker exactly which CVEs to look for.

The OWASP Top 10 in Practical Terms

The OWASP Top 10 isn't a testing checklist so much as a prioritized map of where vulnerabilities are most likely to live. For PHP applications, the most important ones to understand deeply are:

A01 — Broken Access Control: Can user A access user B's invoices by changing an ID in the URL? This is by far the most common vulnerability I find in homegrown billing apps. Test it by creating two accounts and trying to access the second account's resources while authenticated as the first.

A03 — Injection (SQL, LDAP, Command): Any user-supplied input that reaches a query without proper parameterization. In PHP the fix is PDO prepared statements — if I see mysqli_query($conn, "SELECT * FROM invoices WHERE id=" . $_GET['id']) anywhere, that's an immediate finding.

A07 — Identification and Authentication Failures: Weak session tokens, no lockout on login attempts, password reset tokens that don't expire, tokens sent in GET parameters that end up in server logs.

A08 — Software and Data Integrity Failures: Deserializing untrusted data (PHP's unserialize() is a classic attack vector), auto-update mechanisms that don't verify signatures.

Tools: Burp Suite, OWASP ZAP, and Nikto

Burp Suite Community Edition is free and is the industry standard for web application testing. Its proxy intercepts every request your browser makes, lets you modify parameters before they hit the server, and its Repeater tool lets you replay modified requests. This is how I test SQL injection, XSS, and parameter manipulation manually. The Professional edition adds an automated scanner, but for methodical manual testing the free version is sufficient to start.

OWASP ZAP (Zed Attack Proxy) is the fully open-source alternative. I use it specifically for its automated scanner on the initial sweep — it catches low-hanging fruit like missing security headers, directory listing enabled, and some common injection points without any manual configuration. Run ZAP's "Active Scan" on the application first to find the easy wins, then switch to Burp for manual deep testing.

Nikto is a command-line web server scanner that checks for several thousand known issues — outdated server software, dangerous default files, misconfigurations:

nikto -h https://target.com -ssl -output nikto-report.txt

Nikto is noisy (it generates a lot of 404s and is easily spotted in logs) but it's fast and gives you a good initial picture. Never run it on a production system without explicit authorization.

Testing a PHP Application: SQL Injection

For manual SQL injection testing, I start with Burp's Proxy and work through every parameter that reaches the database — GET parameters, POST body fields, cookie values, and HTTP headers like X-Forwarded-For (which some apps log directly to the database without sanitization). The simplest initial test is appending a single quote to a parameter value and observing whether the application throws a database error:

GET /invoices?id=42'  HTTP/1.1

A MySQL error in the response is a confirmed injection point. A blank page or generic error with nothing in the response might be a blind injection — test further with boolean payloads like 42 AND 1=1 (should behave normally) vs 42 AND 1=2 (should return no results). Time-based payloads (42 AND SLEEP(5)) confirm blind injection even when output is completely suppressed.

XSS, CSRF, and Broken Auth Testing

For reflected XSS, I inject a simple payload into every user-supplied field that might be reflected in the response:

<script>alert(document.domain)</script>

If the alert fires, the application is not encoding output properly. For stored XSS, submit the payload in fields that are stored in the database and displayed to other users — comment fields, name fields, notes fields in billing records are frequent culprits.

CSRF testing means checking whether state-changing requests (POST to change a password, POST to cancel a subscription) include and validate a CSRF token. In Burp, intercept a legitimate state-changing request, remove the CSRF token from the POST body, and replay it. If the server accepts the request, CSRF protection is absent or broken.

For authentication testing, I focus on session token entropy (capture 10 session tokens after login and look for patterns), session fixation (set a session ID before login and check if it's regenerated after), and password reset flows (are tokens single-use? do they expire? are they bound to the requesting user's account?).

Writing a Pentest Report

A pentest report that just lists CVEs and tool output is not useful to anyone. The reports I write have four sections: an executive summary (two paragraphs, no technical jargon, describes overall risk posture and the most critical finding), a findings table (each finding gets a title, severity, affected URL/parameter, description, evidence screenshot or request dump, and remediation recommendation), a methodology section (what you tested, what you didn't test, tools used), and a remediation tracking section with a target date column that gets filled in after the client review.

For findings I rate severity on a four-point scale: Critical (direct path to data breach or system compromise), High (significant business impact but requires specific conditions), Medium (limited impact or difficult to exploit), Low (defense-in-depth improvement, no direct exploitation path). Be conservative with Critical — if you're assigning Critical to five different findings, the client stops taking any of them seriously.

Working through this framework on my billing software has made the codebase genuinely more secure over time. It's not a one-time exercise — I run a lightweight version of this on every major feature release, specifically targeting the new attack surface. The discipline of thinking like an attacker before shipping makes for better code from the first draft.