What are the considerations when working with multiple databases or tables in PHP?

When working with multiple databases or tables in PHP, it is important to establish separate database connections for each database and handle them appropriately. This can be done by creating multiple instances of the `PDO` class, each pointing to a different database, and executing queries on the respective database connection. 

// Establish connection to first database
$dsn1 = 'mysql:host=localhost;dbname=database1';
$username1 = 'username';
$password1 = 'password';
$pdo1 = new PDO($dsn1, $username1, $password1);

// Establish connection to second database
$dsn2 = 'mysql:host=localhost;dbname=database2';
$username2 = 'username';
$password2 = 'password';
$pdo2 = new PDO($dsn2, $username2, $password2);

// Query database1
$stmt1 = $pdo1->query('SELECT * FROM table1');
while ($row = $stmt1->fetch()) {
    // Process data from table1
}

// Query database2
$stmt2 = $pdo2->query('SELECT * FROM table2');
while ($row = $stmt2->fetch()) {
    // Process data from table2
}

Keywords

MySQL PDO database connections SQL queries data mapping

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