How can different types of quotes (' vs ") impact the execution of SQL queries in PHP?

When writing SQL queries in PHP, using single quotes ('') or double quotes ("") can impact the execution of the query. Single quotes are used to enclose string literals in SQL, while double quotes are used for identifiers like table or column names. Mixing up these quotes can result in syntax errors or unexpected behavior in SQL queries. To avoid this issue, always use the appropriate quote type for string literals and identifiers in your SQL queries.

// Example of using single quotes for string literals and double quotes for identifiers in an SQL query
$query = "SELECT * FROM users WHERE username = 'john_doe'";