PHP PDO::__construct Method

last modified April 19, 2025

The PDO::__construct method creates a new PDO instance representing a connection to a database. It is the primary way to establish database connections in PHP using PDO.

Basic Definition

PDO::__construct creates a new PHP Data Objects (PDO) instance. This represents a connection between PHP and a database server. The method takes a DSN (Data Source Name) parameter.

Syntax: public PDO::__construct(string $dsn, string $username = null, string $password = null, array $options = null). The DSN contains the database type, host, and database name.

MySQL Connection Example

This shows how to connect to a MySQL database using PDO::__construct.

pdo_mysql_connect.php

<?php

declare(strict_types=1);

$dsn = 'mysql:host=localhost;dbname=testdb;charset=utf8mb4';
$username = 'dbuser';
$password = 'dbpass';

try {
    $pdo = new PDO($dsn, $username, $password);
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected to MySQL successfully";
} catch (PDOException $e) {
    die("Connection failed: " . $e->getMessage());
}

This creates a connection to a MySQL database. The DSN specifies the host, database name, and charset. We set the error mode to exceptions for better error handling.

SQLite Connection Example

This demonstrates connecting to an SQLite database file using PDO.

pdo_sqlite_connect.php

<?php

declare(strict_types=1);

$databaseFile = 'example.db';

try {
    $pdo = new PDO("sqlite:$databaseFile");
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected to SQLite database successfully";
} catch (PDOException $e) {
    die("Connection failed: " . $e->getMessage());
}

For SQLite, the DSN is simpler - just the driver and database file path. No username or password is required for SQLite file databases.

PostgreSQL Connection Example

This shows how to connect to a PostgreSQL database with PDO.

pdo_pgsql_connect.php

<?php

declare(strict_types=1);

$dsn = 'pgsql:host=localhost;port=5432;dbname=testdb';
$username = 'pguser';
$password = 'pgpass';

try {
    $pdo = new PDO($dsn, $username, $password);
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected to PostgreSQL successfully";
} catch (PDOException $e) {
    die("Connection failed: " . $e->getMessage());
}

The PostgreSQL DSN includes the host, port, and database name. PostgreSQL requires authentication with username and password.

Connection with Options

This demonstrates passing connection options to the PDO constructor.

pdo_options_connect.php

<?php

declare(strict_types=1);

$dsn = 'mysql:host=localhost;dbname=testdb';
$username = 'dbuser';
$password = 'dbpass';
$options = [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::ATTR_EMULATE_PREPARES => false,
];

try {
    $pdo = new PDO($dsn, $username, $password, $options);
    echo "Connected with custom options successfully";
} catch (PDOException $e) {
    die("Connection failed: " . $e->getMessage());
}

The options array configures PDO behavior. We set error mode, fetch mode, and disable prepared statement emulation. Options can also be set later with setAttribute.

Persistent Connection

This shows how to create a persistent database connection with PDO.

pdo_persistent_connect.php

<?php

declare(strict_types=1);

$dsn = 'mysql:host=localhost;dbname=testdb';
$username = 'dbuser';
$password = 'dbpass';
$options = [
    PDO::ATTR_PERSISTENT => true,
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
];

try {
    $pdo = new PDO($dsn, $username, $password, $options);
    echo "Persistent connection established successfully";
} catch (PDOException $e) {
    die("Connection failed: " . $e->getMessage());
}

Persistent connections remain open after script execution, reducing overhead. Use them carefully as they can lead to too many open connections.

Connection with SSL

This demonstrates establishing a secure MySQL connection with SSL.

pdo_ssl_connect.php

<?php

declare(strict_types=1);

$dsn = 'mysql:host=localhost;dbname=testdb';
$username = 'dbuser';
$password = 'dbpass';
$options = [
    PDO::MYSQL_ATTR_SSL_CA => '/path/to/ca-cert.pem',
    PDO::MYSQL_ATTR_SSL_CERT => '/path/to/client-cert.pem',
    PDO::MYSQL_ATTR_SSL_KEY => '/path/to/client-key.pem',
    PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT => false,
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
];

try {
    $pdo = new PDO($dsn, $username, $password, $options);
    echo "Secure SSL connection established successfully";
} catch (PDOException $e) {
    die("Connection failed: " . $e->getMessage());
}

This configures SSL for the MySQL connection. The options specify certificate files and disable server certificate verification (not recommended for production).

Connection with Custom Attributes

This shows setting custom driver-specific attributes during connection.

pdo_custom_attrs.php

<?php

declare(strict_types=1);

$dsn = 'mysql:host=localhost;dbname=testdb';
$username = 'dbuser';
$password = 'dbpass';
$options = [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8mb4",
    PDO::MYSQL_ATTR_USE_BUFFERED_QUERY => true,
    PDO::ATTR_TIMEOUT => 5
];

try {
    $pdo = new PDO($dsn, $username, $password, $options);
    echo "Connection with custom attributes established";
} catch (PDOException $e) {
    die("Connection failed: " . $e->getMessage());
}

This sets MySQL-specific attributes like INIT_COMMAND and USE_BUFFERED_QUERY. The ATTR_TIMEOUT sets a connection timeout. Different drivers support different custom attributes.

Best Practices

DSN Format: Always use the correct DSN format for your database.
Error Handling: Always use try-catch blocks with PDO.
Secure Credentials: Store credentials securely, not in code.
Character Set: Specify charset in DSN or with INIT_COMMAND.
Connection Options: Set important options during connection.

Source

PHP PDO::__construct Documentation

This tutorial covered the PDO::__construct method with practical examples showing different database connections and configuration options.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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