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Stop Writing Raw PHP Queries — Prevent SQL Injection Now

Stop Writing Raw PHP Queries — Prevent SQL Injection Now

Meta description: I learned to prevent SQL injection in raw PHP queries the hard way after a failed audit. Here’s exactly how I fixed every vulnerable query.

Last updated: June 12, 2026

A few years ago, a security audit flagged 14 endpoints in a legacy PHP app I inherited — all vulnerable to SQL injection. The code used string concatenation to build queries directly from $_GET and $_POST values. Fixing all 14 took a weekend, but it taught me exactly how SQL injection happens and how to prevent it permanently.

TL;DR

  • Always use prepared statements with PDO or mysqli, never string concatenation
  • Bind user input as parameters — never insert variables directly into SQL strings
  • Use allow-lists for dynamic identifiers like column names and table names, since those can’t be parameterized

Why SQL Injection Prevention Matters in Raw PHP

SQL injection remains one of the most common vulnerabilities in PHP applications, especially in legacy codebases written before PDO became standard. When user input is concatenated directly into a query string, an attacker can inject SQL syntax that changes the query’s logic entirely.

In my experience, the most dangerous queries aren’t the obvious login forms — they’re the “small” admin tools, search filters, and report generators that developers assume only internal staff will use.

Important: “Sanitizing” input with functions like addslashes() or mysql_real_escape_string() is not sufficient protection. These are legacy approaches that miss edge cases — prepared statements are the only reliable fix.

Prerequisites

To follow along, you’ll need:

  • PHP 8.0 or higher (I’m using PHP 8.2 in these examples)
  • A MySQL or MariaDB database
  • The pdo_mysql extension enabled (php -m | grep pdo_mysql to check)
  • Basic familiarity with PHP’s database connection functions

Step-by-Step Implementation

Step 1: Identify Vulnerable Queries

The pattern I look for is any query built with string concatenation or interpolation involving $_GET, $_POST, $_REQUEST, or $_COOKIE:

// VULNERABLE - never do this
$id = $_GET['id'];
$result = $conn->query("SELECT * FROM users WHERE id = $id");

An attacker could pass ?id=1 OR 1=1 and dump the entire users table.

Step 2: Switch to PDO with Prepared Statements

Here’s the same query rewritten safely:

$pdo = new PDO('mysql:host=localhost;dbname=myapp', $user, $pass, [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_EMULATE_PREPARES => false,
]);

$stmt = $pdo->prepare("SELECT * FROM users WHERE id = :id");
$stmt->execute(['id' => $_GET['id']]);
$result = $stmt->fetch();

Setting PDO::ATTR_EMULATE_PREPARES to false is important — without it, some drivers fall back to client-side emulation, which has historically had edge-case vulnerabilities with certain character encodings.

Step 3: Handle Dynamic Column and Table Names Safely

Prepared statement placeholders only work for values, not identifiers like column or table names. If your code builds a query like ORDER BY $sortColumn, you can’t parameterize that directly.

$allowedColumns = ['name', 'email', 'created_at'];
$sortColumn = in_array($_GET['sort'], $allowedColumns, true) ? $_GET['sort'] : 'name';

$stmt = $pdo->prepare("SELECT * FROM users ORDER BY $sortColumn");
$stmt->execute();

The allow-list check is the critical step — without it, $sortColumn is still attacker-controlled.

Step 4: Apply the Same Pattern with mysqli

If your codebase uses mysqli instead of PDO:

$stmt = $conn->prepare("SELECT * FROM users WHERE email = ?");
$stmt->bind_param("s", $_POST['email']);
$stmt->execute();
$result = $stmt->get_result();

The "s" tells mysqli the bound parameter is a string, which is required for bind_param to work correctly.

[INTERNAL LINK: related article]

Real-World Tips I Use in Production

When I audited that legacy app, here’s what made the process manageable:

  • I wrote a quick regex search (grep -rn '\$_GET\|\$_POST' --include=*.php) to find every file touching superglobals, then cross-referenced which ones built SQL strings
  • I created a small wrapper class around PDO so every query in the app went through the same prepared-statement interface, making it impossible to “accidentally” concatenate again
  • For search features using LIKE, I bind the wildcard characters into the parameter itself: bindValue(':term', '%' . $term . '%') rather than building the string with wildcards inline

Pro Tip: Enable PHP’s error_reporting to catch type mismatches in bind_param during development — a mismatched type string is a common source of silent failures.

Common Errors and How I Fixed Them

The first error I ran into during the migration was:

Fatal error: Uncaught Error: Call to a member function bind_param() on bool

This happens when prepare() returns false, usually because of a SQL syntax error in the query string itself — in my case, I had a typo using a reserved word order as a column name without backticks. The fix was wrapping it: SELECT * FROM users ORDER BY `order`.

The second issue was queries silently returning no rows after switching to prepared statements. The cause was passing an integer where PDO expected a string parameter type — switching to named parameters and explicit bindValue() calls with PDO::PARAM_INT resolved it.

Frequently Asked Questions

Q: How do I prevent SQL injection in PHP without using a framework? A: Use PDO or mysqli prepared statements with bound parameters for all user input — no framework is required for this protection.

Q: Is mysqli or PDO better for preventing SQL injection? A: Both prevent SQL injection equally well when used with prepared statements; PDO is generally preferred for its support of multiple database drivers.

Q: Can prepared statements protect column and table names from SQL injection? A: No — prepared statement placeholders only work for values, so column and table names must be validated against an allow-list.

Q: Does addslashes() prevent SQL injection in PHP? A: No, addslashes() is not a reliable defense against SQL injection and should be replaced with prepared statements.

Q: How can I find SQL injection vulnerabilities in an existing PHP codebase? A: Search for raw queries built with string concatenation or interpolation involving superglobals like $_GET and $_POST, then refactor them to use prepared statements.

About the Author

I’m a backend developer with over 8 years of experience working primarily in PHP, MySQL, and security-focused code audits. I’ve led remediation efforts on legacy codebases moving from raw queries to PDO-based prepared statements. I write about practical security fixes developers can apply immediately, not just theory.

[SOURCE: https://owasp.org/www-community/attacks/SQL_Injection] [SOURCE: https://www.php.net/manual/en/pdo.prepared-statements.php]