What are the advantages of using a whitelist approach over a blacklist approach when filtering out unwanted characters in PHP?

When filtering out unwanted characters in PHP, using a whitelist approach is generally considered more secure and effective than a blacklist approach. A whitelist approach involves specifying the allowed characters to be included, while a blacklist approach involves specifying the characters to be excluded. By using a whitelist approach, you can ensure that only the desired characters are accepted, reducing the risk of allowing unexpected or malicious input.

// Whitelist approach to filter out unwanted characters
$input = "Hello123!";
$filtered_input = preg_replace("/[^a-zA-Z0-9]/", "", $input);
echo $filtered_input; // Output: Hello123