PHP's FILTER_SANITIZE_STRING Deprecation: What Are the Best Replacement Options?

FILTER_SANITIZE_STRING Deprecation: A Tale of Confusion and Unintended Consequences

The recent deprecation of the FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED constants has raised concerns among PHP developers who rely on them for input sanitization.

What's Deprecated and Why?

FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED were previously used to remove potential XSS vulnerabilities from input strings. However, these filters exhibited confusing and unintuitive behavior. FILTER_SANITIZE_STRING stripped all characters between '<' and the end of the string, removed NUL bytes, and encoded single and double quotes.

The PHP community determined that these filters caused more confusion than they solved, as developers often misunderstood their intended use. Input sanitization is already handled adequately by other filters, such as FILTER_UNSAFE_RAW.

Replacement Options

There are several options for replacing these deprecated filters:

• FILTER_UNSAFE_RAW: This default string filter does not perform any filtering. Use this if you want the raw string value without any modifications.
• htmlspecialchars(): Use this function to encode special characters that could be exploited for XSS vulnerabilities. However, remember to apply it to the output, not the input.
• Custom Polyfill: For those who require the specific filtering behavior of FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED, a regex-based polyfill can be created as follows:

function filter_string_polyfill(string $string): string
{
    $str = preg_replace('/\x00|<[^>]*>?/', '', $string);
    return str_replace(["'", '"'], ['&#39;', '&#34;'], $str);
}

Remember the Golden Rule
It's crucial to emphasize that input sanitization should not be considered a reliable defense against XSS attacks. Instead, developers should focus on escaping output to prevent potentially dangerous content from being injected into the page.
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