What are the Best Replacements for the Deprecated PHP FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED Constants?

Understanding the Depreciation Notice: Constant FILTER_SANITIZE_STRING is Deprecated

In PHP 8.1, the constant FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED have been deprecated. This raises the question: what are suitable replacements for these constants?

Depreciation Rationale

These filters were imprecise in their purpose and could lead to confusion. Their functionality was often misapprehended as the default string filter, FILTER_UNSAFE_RAW. As a result, the PHP community has deprecated their usage.

Replacement Options

There are several options available for replacing FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED:

• FILTER_UNSAFE_RAW: This filter applies no filtering and can be used if no specific sanitization is required.
• htmlspecialchars(): Use this function to protect against XSS vulnerabilities by encoding output, rather than sanitizing input.
• Custom Regex Polyfill: For cases where specific functionality is required, a regex polyfill can be created to replicate the behavior of the deprecated filters:

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

Best Practice: Output Escaping

It's important to remember that sanitizing input is not effective in preventing security risks. Instead, focus on escaping output to protect against XSS vulnerabilities. This ensures that the output is safe regardless of any malicious input that may have been received.

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