How can missing periods in PHP code affect the functionality of SQL queries?

Missing periods in PHP code can affect the functionality of SQL queries by causing syntax errors or unexpected behavior. It is important to correctly concatenate strings and variables with periods to ensure the SQL query is formed correctly. 

// Incorrect way of concatenating strings and variables in SQL query
$sql = "SELECT * FROM users WHERE id = $userId AND name = '$userName'";

// Correct way of concatenating strings and variables in SQL query
$sql = "SELECT * FROM users WHERE id = " . $userId . " AND name = '" . $userName . "'";

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