[Solution] PHP mysql_* Deprecated — Replace with mysqli_* Migration

The original mysql_* extension was deprecated in PHP 5.5 and removed in PHP 7.0. It was replaced by mysqli_* (MySQL Improved) and PDO (PHP Data Objects), both of which support prepared statements, which are critical for preventing SQL injection. Migrating is mandatory — your code will not run on PHP 7.0+ without updating.

What You’ll See

On PHP 7.0+:

Fatal error: Uncaught Error: Call to undefined function mysql_connect()

On PHP 5.5 through 5.6 (deprecated mode):

Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in /path/to/script.php on line X

Why Deprecated

The mysql_* extension was removed for several critical reasons:

No prepared statements: Without prepared statements, every query required manual escaping, which developers frequently forgot, leading to SQL injection vulnerabilities.
Not maintained: The extension had not received updates for years and was incompatible with modern MySQL features.
Security: Prepared statements via mysqli or PDO provide parametrized queries that inherently prevent injection attacks.
Features: mysqli supports transactions, multiple statements, and improved error handling.

Old Code (Deprecated)

// Connection
$conn = mysql_connect("localhost", "user", "password");
mysql_select_db("mydb", $conn);

// Simple query
$result = mysql_query("SELECT * FROM users WHERE id = 1", $conn);
$row = mysql_fetch_assoc($result);
echo $row['name'];

// Insert with manual escaping (SQL injection risk)
$name = $_POST['name'];
$query = "INSERT INTO users (name) VALUES ('" . mysql_real_escape_string($name) . "')";
mysql_query($query, $conn);

// Row count
$count = mysql_num_rows($result);

// Free result
mysql_free_result($result);

// Close connection
mysql_close($conn);

New Code — mysqli_* Approach

// Connection (object-oriented)
$conn = new mysqli("localhost", "user", "password", "mydb");

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

// Simple query
$result = $conn->query("SELECT * FROM users WHERE id = 1");
$row = $result->fetch_assoc();
echo $row['name'];

// Insert with prepared statement (SQL injection safe)
$stmt = $conn->prepare("INSERT INTO users (name) VALUES (?)");
$stmt->bind_param("s", $_POST['name']);
$stmt->execute();
$stmt->close();

// Row count
$count = $result->num_rows;

// Free result
$result->free();

// Close connection
$conn->close();

New Code — PDO Approach (Recommended)

// Connection
$pdo = new PDO(
    "mysql:host=localhost;dbname=mydb;charset=utf8mb4",
    "user",
    "password",
    [
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
        PDO::ATTR_EMULATE_PREPARES => false,
    ]
);

// Simple query
$stmt = $pdo->query("SELECT * FROM users WHERE id = 1");
$row = $stmt->fetch();
echo $row['name'];

// Insert with prepared statement
$stmt = $pdo->prepare("INSERT INTO users (name) VALUES (:name)");
$stmt->execute(['name' => $_POST['name']]);

// Fetch all rows
$stmt = $pdo->query("SELECT * FROM users WHERE active = 1");
$rows = $stmt->fetchAll();

// Using transaction
$pdo->beginTransaction();
try {
    $pdo->exec("UPDATE accounts SET balance = balance - 100 WHERE id = 1");
    $pdo->exec("UPDATE accounts SET balance = balance + 100 WHERE id = 2");
    $pdo->commit();
} catch (Exception $e) {
    $pdo->rollBack();
    throw $e;
}

Migration Steps

Find all mysql_ calls*:

grep -rn "\bmysql_" --include="*.php" /path/to/project/

Choose your replacement: mysqli is a closer drop-in match, but PDO is the modern standard and supports multiple database drivers.
Replace the connection with either new mysqli() or new PDO().
Convert every query that uses user input to a prepared statement with bound parameters. This is the most important step for security.
Replace row-count functions: mysql_num_rows($result) becomes $result->num_rows (mysqli) or $stmt->rowCount() (PDO).
Replace mysql_fetch_* calls with $result->fetch_assoc() (mysqli) or $stmt->fetch() (PDO).
Update error handling. mysqli returns false on failure; PDO can throw exceptions with ERRMODE_EXCEPTION.
Test every database interaction in your application. This is a high-risk migration — do not skip testing.

For very large codebases, consider using Rector to automate the mechanical parts of the conversion.