What are common syntax errors to watch out for when working with PHP variables in file names?

Common syntax errors to watch out for when working with PHP variables in file names include not properly concatenating variables with strings using the dot (.) operator, not enclosing variables in double quotes when using them in file paths, and using special characters that are not allowed in file names. To avoid these errors, make sure to concatenate variables correctly, enclose them in double quotes when needed, and sanitize the variables to ensure they do not contain any invalid characters. Example PHP code snippet:

// Incorrect way of using variables in file names
$fileName = "file_" . $variable . ".txt";

// Correct way of using variables in file names
$fileName = "file_" . $variable . ".txt";
// or
$fileName = "file_{$variable}.txt";