What are the differences between using single quotes and double quotes in PHP when constructing SQL queries?

When constructing SQL queries in PHP, using single quotes and double quotes can affect how variables are interpreted within the query. Single quotes treat variables as literal strings, while double quotes allow for variable interpolation. To ensure proper variable substitution in SQL queries, it is recommended to use double quotes when enclosing the query and single quotes for string values within the query.

// Using double quotes to allow for variable interpolation
$query = "SELECT * FROM users WHERE username = '$username'";