What's the Best Way to Replace the Deprecated PHP FILTER_SANITIZE_STRING?

Deprecation of Constant FILTER_SANITIZE_STRING

With the release of PHP 8.1, the constant FILTER_SANITIZE_STRING has been deprecated. This has raised concerns among developers who previously relied on this filter for sanitizing string inputs.

Why was FILTER_SANITIZE_STRING Deprecated?

According to the PHP community, FILTER_SANITIZE_STRING had an unclear purpose and could lead to confusion. It removed arbitrary content without providing a clear explanation of its functionality. Additionally, it has been superseded by more effective string sanitization techniques.

What Can You Replace It With?

There are several options to replace the deprecated FILTER_SANITIZE_STRING:

• FILTER_UNSAFE_RAW: This is the default string filter and provides no filtering. It can be used if no sanitization is desired.
• htmlspecialchars(): This function encodes special characters to prevent cross-site scripting (XSS) vulnerabilities. It should be used to encode output data, not input data.
• Custom Polyfill: If you need to recreate the functionality of FILTER_SANITIZE_STRING, you can create a custom polyfill using regular expressions.

Example Polyfill

function filter_string_polyfill(string $string): string
{
    $str = preg_replace('/\x00|<[^>]*>?/', '', $string);
    return str_replace(["'", '"'], [''', '"'], $str);
}

Best Practice: Escape Output

It's important to emphasize that sanitization should be applied to output data, not input data. By escaping special characters in the output, you prevent malicious scripts or data from affecting your application.

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