apply_filters( 'sanitize_key', string $key, string $raw_key )

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• By default, sanitize_key() function sanitize a string key, which is used as internal identifiers, into lowercase alphanumeric, dashes, and underscores characters. After sanitize_key() function has done its work, it passes the sanitized key through this sanitize_key filter.
• The filter passes $key and $raw_key as parameters, which allows the user of this filter to perform additional sanitization based on these two keys.