When developing in PHP, you may encounter common mistakes. Some of these mistakes are simple, while others can be time-consuming and complicate your development process.

Here are the most frequent PHP mistakes and their solutions:

1. Undefined Variable Error

Error: Trying to use an undefined variable.

echo $name;

Why Does It Happen? PHP does not throw an error but shows a “Notice” warning when trying to use an undefined variable.

Solution: Always define variables before using them.

$name = "Ali";
echo $name;

Or check if the variable exists:

if (isset($name)) {
    echo $name;
} else {
    echo "Variable is not defined!";
}

2. Headers Already Sent Error

Error: Trying to use header() after sending output to the browser.

echo "Welcome!";
header("Location: homepage.php");

Why Does It Happen? PHP cannot modify HTTP headers after outputting any content.

Solution: Always use header() before any output:

header("Location: homepage.php");
exit;
echo "Welcome!";

Alternatively, use output buffering:

ob_start();
echo "Welcome!";
header("Location: homepage.php");
ob_end_flush();

3. Vulnerability to SQL Injection

Error: Using raw user input in SQL queries.

$query = "SELECT * FROM users WHERE username = '" . $_GET['username'] . "'";

Why Does It Happen? This allows malicious users to inject harmful SQL queries.

Solution: Always use PDO or mysqli_real_escape_string() to sanitize input:

$stmt = $db->prepare("SELECT * FROM users WHERE username = ?");
$stmt->execute([$_GET['username']]);
$result = $stmt->fetch();