How does a poorly designed database structure impact the performance and scalability of PHP applications, and what steps can be taken to rectify this issue?

A poorly designed database structure can impact the performance and scalability of PHP applications by causing slow query execution, inefficient data retrieval, and increased server load. To rectify this issue, steps can be taken such as optimizing database indexes, normalizing data tables, and using efficient SQL queries.

// Example code snippet for optimizing database indexes in PHP
$sql = "CREATE INDEX index_name ON table_name (column_name)";
$result = mysqli_query($connection, $sql);
if($result){
    echo "Index created successfully";
} else {
    echo "Error creating index: " . mysqli_error($connection);
}