What are common pitfalls when using htmlentities() and trim() for form validation in PHP?

Common pitfalls when using htmlentities() and trim() for form validation in PHP include: 1. Using htmlentities() for form validation can lead to data loss as it converts special characters to HTML entities, which may not be desired for all form fields. 2. Using trim() alone may not adequately sanitize user input, as it only removes whitespace from the beginning and end of a string, leaving other potentially harmful characters untouched. To address these issues, it is recommended to use htmlentities() for output escaping to prevent XSS attacks, and a combination of trim() and filter_var() with appropriate filters for input validation.

// Example of using filter_var() with trim() for form validation
$input = $_POST['input_field'];

// Trim whitespace and sanitize input
$trimmed_input = trim($input);
$filtered_input = filter_var($trimmed_input, FILTER_SANITIZE_STRING);

if ($filtered_input !== $trimmed_input) {
    // Input contains invalid characters, handle error
} else {
    // Proceed with sanitized input
}