ull $encoding Optional. Character encoding to use. Default null.
 * @return string Extracted substring.
 */
function _mb_substr( $str, $start, $length = null, $encoding = null ) {
	if ( null === $str ) {
		return '';
	}

	// The solution below works only for UTF-8; treat all other encodings as byte streams.
	if ( ! _is_utf8_charset( $encoding ?? get_option( 'blog_charset' ) ) ) {
		$result = is_null( $length ) ? substr( $str, $start ) : substr( $str, $start, $length );

		/*
		 * For an out-of-range start, substr() returns false on PHP < 8.0 but an
		 * empty string on PHP >= 8.0. mb_substr() always returns an empty string,
		 * so normalize to match its behavior across all supported PHP versions.
		 */
		return false === $result ? '' : $result;
	}

	$total_length = ( $start < 0 || $length < 0 )
		? _wp_utf8_codepoint_count( $str )
		: 0;

	$normalized_start = $start < 0
		? max( 0, $total_length + $start )
		: $start;

	/*
	 * The starting offset is provided as characters, which means this needs to
	 * find how many bytes that many characters occupies at the start of the string.
	 */
	$starting_byte_offset = _wp_utf8_codepoint_span( $str, 0, $normalized_start );

	$normalized_length = $length < 0
		? max( 0, $total_length - $normalized_start + $length )
		: $length;

	/*
	 * This is the main step. It finds how many bytes the given length of code points
	 * occupies in the input, starting at the byte offset calculated above.
	 */
	$byte_length = isset( $normalized_length )
		? _wp_utf8_codepoint_span( $str, $starting_byte_offset, $normalized_length )
		: ( strlen( $str ) - $starting_byte_offset );

	// The result is a normal byte-level substring using the computed ranges.
	return substr( $str, $starting_byte_offset, $byte_length );
}

if ( ! function_exists( 'mb_strlen' ) ) :
	/**
	 * Compat function to mimic mb_strlen().
	 *
	 * @ignore
	 * @since 4.2.0
	 *
	 * @see _mb_strlen()
	 *
	 * @param string      $string   The string to retrieve the character length from.
	 * @param string|null $encoding Optional. Character encoding to use. Default null.
	 * @return int String length of `$string`.
	 */
	function mb_strlen( $string, $encoding = null ) { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.stringFound
		return _mb_strlen( $string, $encoding );
	}
endif;

/**
 * Internal compat function to mimic mb_strlen().
 *
 * Only supports UTF-8 and non-shifting single-byte encodings. For all other
 * encodings expect the counts to be wrong. When the given encoding (or the
 * `blog_charset` if none is provided) isn’t UTF-8 then the function returns
 * the byte-count of the provided string.
 *
 * @ignore
 * @since 4.2.0
 *
 * @param string      $str      The string to retrieve the character length from.
 * @param string|null $encoding Optional. Count characters according to this encoding.
 *                              Default is to consult `blog_charset`.
 * @return int Count of code points if UTF-8, byte length otherwise.
 */
function _mb_strlen( $str, $encoding = null ) {
	return _is_utf8_charset( $encoding ?? get_option( 'blog_charset' ) )
		? _wp_utf8_codepoint_count( $str )
		: strlen( $str );
}

if ( ! function_exists( 'utf8_encode' ) ) :
	if ( extension_loaded( 'mbstring' ) ) :
		/**
		 * Converts a string from ISO-8859-1 to UTF-8.
		 *
		 * @deprecated Use {@see \mb_convert_encoding()} instead.
		 *
		 * @since 6.9.0
		 *
		 * @param string $iso_8859_1_text Text treated as ISO-8859-1 (latin1) bytes.
		 * @return string Text converted into a UTF-8.
		 */
		function utf8_encode( $iso_8859_1_text ): string {
			_deprecated_function( __FUNCTION__, '6.9.0', 'mb_convert_encoding' );

			return mb_convert_encoding( $iso_8859_1_text, 'UTF-8', 'ISO-8859-1' );
		}

	else :
		/**
		 * @ignore
		 * @private
		 *
		 * @since 6.9.0
		 */
		function utf8_encode( $iso_8859_1_text ): string {
			_deprecated_function( __FUNCTION__, '6.9.0', 'mb_convert_encoding' );

			return _wp_utf8_encode_fallback( $iso_8859_1_text );
		}

	endif;
endif;

if ( ! function_exists( 'utf8_decode' ) ) :
	if ( extension_loaded( 'mbstring' ) ) :
		/**
		 * Converts a string from UTF-8 to ISO-8859-1.
		 *
		 * @deprecated Use {@see \mb_convert_encoding()} instead.
		 *
		 * @since 6.9.0
		 *
		 * @param string $utf8_text Text treated as UTF-8.
		 * @return string Text converted into ISO-8859-1.
		 */
		function utf8_decode( $utf8_text ): string {
			_deprecated_function( __FUNCTION__, '6.9.0', 'mb_convert_encoding' );

			return mb_convert_encoding( $utf8_text, 'ISO-8859-1', 'UTF-8' );
		}

	else :
		/**
		 * @ignore
		 * @private
		 *
		 * @since 6.9.0
		 */
		function utf8_decode( $utf8_text ): string {
			_deprecated_function( __FUNCTION__, '6.9.0', 'mb_convert_encoding' );

			return _wp_utf8_decode_fallback( $utf8_text );
		}

	endif;
endif;

// sodium_crypto_box() was introduced with Sodium in PHP 7.2, but the extension may not be enabled.
if ( ! function_exists( 'sodium_crypto_box' ) ) {
	require ABSPATH . WPINC . '/sodium_compat/autoload.php';
}

if ( ! function_exists( 'array_is_list' ) ) {
	/**
	 * Polyfill for `array_is_list()` function added in PHP 8.1.
	 *
	 * Determines if the given array is a list.
	 *
	 * An array is considered a list if its keys consist of consecutive numbers from 0 to count($array)-1.
	 *
	 * @see https://github.com/symfony/polyfill-php81/tree/main
	 *
	 * @since 6.5.0
	 *
	 * @param array<mixed> $arr The array being evaluated.
	 * @return bool True if array is a list, false otherwise.
	 */
	function array_is_list( $arr ) {
		if ( ( array() === $arr ) || ( array_values( $arr ) === $arr ) ) {
			return true;
		}

		$next_key = -1;

		foreach ( $arr as $k => $v ) {
			if ( ++$next_key !== $k ) {
				return false;
			}
		}

		return true;
	}
}

if ( ! function_exists( 'str_contains' ) ) {
	/**
	 * Polyfill for `str_contains()` function added in PHP 8.0.
	 *
	 * Performs a case-sensitive check indicating if needle is
	 * contained in haystack.
	 *
	 * @since 5.9.0
	 *
	 * @param string $haystack The string to search in.
	 * @param string $needle   The substring to search for in the `$haystack`.
	 * @return bool True if `$needle` is in `$haystack`, otherwise false.
	 */
	function str_contains( $haystack, $needle ) {
		if ( '' === $needle ) {
			return true;
		}

		return false !== strpos( $haystack, $needle );
	}
}

if ( ! function_exists( 'str_starts_with' ) ) {
	/**
	 * Polyfill for `str_starts_with()` function added in PHP 8.0.
	 *
	 * Performs a case-sensitive check indicating if
	 * the haystack begins with needle.
	 *
	 * @since 5.9.0
	 *
	 * @param string $haystack The string to search in.
	 * @param string $needle   The substring to search for in the `$haystack`.
	 * @return bool True if `$haystack` starts with `$needle`, otherwise false.
	 */
	function str_starts_with( $haystack, $needle ) {
		if ( '' === $needle ) {
			return true;
		}

		return 0 === strpos( $haystack, $needle );
	}
}

if ( ! function_exists( 'str_ends_with' ) ) {
	/**
	 * Polyfill for `str_ends_with()` function added in PHP 8.0.
	 *
	 * Performs a case-sensitive check indicating if
	 * the haystack ends with needle.
	 *
	 * @since 5.9.0
	 *
	 * @param string $haystack The string to search in.
	 * @param string $needle   The substring to search for in the `$haystack`.
	 * @return bool True if `$haystack` ends with `$needle`, otherwise false.
	 */
	function str_ends_with( $haystack, $needle ) {
		if ( '' === $haystack ) {
			return '' === $needle;
		}

		$len = strlen( $needle );

		return substr( $haystack, -$len, $len ) === $needle;
	}
}

if ( ! function_exists( 'array_find' ) ) {
	/**
	 * Polyfill for `array_find()` function added in PHP 8.4.
	 *
	 * Searches an array for the first element that passes a given callback.
	 *
	 * @since 6.8.0
	 *
	 * @param array    $array    The array to search.
	 * @param callable $callback The callback to run for each element.
	 * @return mixed|null The first element in the array that passes the `$callback`, otherwise null.
	 */
	function array_find( array $array, callable $callback ) { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.arrayFound
		foreach ( $array as $key => $value ) {
			if ( $callback( $value, $key ) ) {
				return $value;
			}
		}

		return null;
	}
}

if ( ! function_exists( 'array_find_key' ) ) {
	/**
	 * Polyfill for `array_find_key()` function added in PHP 8.4.
	 *
	 * Searches an array for the first key that passes a given callback.
	 *
	 * @since 6.8.0
	 *
	 * @param array    $array    The array to search.
	 * @param callable $callback The callback to run for each element.
	 * @return int|string|null The first key in the array that passes the `$callback`, otherwise null.
	 */
	function array_find_key( array $array, callable $callback ) { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.arrayFound
		foreach ( $array as $key => $value ) {
			if ( $callback( $value, $key ) ) {
				return $key;
			}
		}

		return null;
	}
}

if ( ! function_exists( 'array_any' ) ) {
	/**
	 * Polyfill for `array_any()` function added in PHP 8.4.
	 *
	 * Checks if any element of an array passes a given callback.
	 *
	 * @since 6.8.0
	 *
	 * @param array    $array    The array to check.
	 * @param callable $callback The callback to run for each element.
	 * @return bool True if any element in the array passes the `$callback`, otherwise false.
	 */
	function array_any( array $array, callable $callback ): bool { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.arrayFound
		foreach ( $array as $key => $value ) {
			if ( $callback( $value, $key ) ) {
				return true;
			}
		}

		return false;
	}
}

if ( ! function_exists( 'array_all' ) ) {
	/**
	 * Polyfill for `array_all()` function added in PHP 8.4.
	 *
	 * Checks if all elements of an array pass a given callback.
	 *
	 * @since 6.8.0
	 *
	 * @param array    $array    The array to check.
	 * @param callable $callback The callback to run for each element.
	 * @return bool True if all elements in the array pass the `$callback`, otherwise false.
	 */
	function array_all( array $array, callable $callback ): bool { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.arrayFound
		foreach ( $array as $key => $value ) {
			if ( ! $callback( $value, $key ) ) {
				return false;
			}
		}

		return true;
	}
}

if ( ! function_exists( 'array_first' ) ) {
	/**
	 * Polyfill for `array_first()` function added in PHP 8.5.
	 *
	 * Returns the first element of an array.
	 *
	 * @since 6.9.0
	 *
	 * @param array $array The array to get the first element from.
	 * @return mixed|null The first element of the array, or null if the array is empty.
	 */
	function array_first( array $array ) { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.arrayFound
		if ( empty( $array ) ) {
			return null;
		}

		foreach ( $array as $value ) {
			return $value;
		}
	}
}

if ( ! function_exists( 'array_last' ) ) {
	/**
	 * Polyfill for `array_last()` function added in PHP 8.5.
	 *
	 * Returns the last element of an array.
	 *
	 * @since 6.9.0
	 *
	 * @param array $array The array to get the last element from.
	 * @return mixed|null The last element of the array, or null if the array is empty.
	 */
	function array_last( array $array ) { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.arrayFound
		if ( empty( $array ) ) {
			return null;
		}

		return $array[ array_key_last( $array ) ];
	}
}

if ( ! function_exists( 'clamp' ) ) {
	/**
	 * Polyfill for `clamp()` function added in PHP 8.6.
	 *
	 * Clamps a value to be within the range of a given minimum and maximum.
	 *
	 * If the value is within the bounds, the original value is returned.
	 * If it is not within the bounds, the closest bound is returned.
	 *
	 * @since 7.1.0
	 *
	 * @param mixed $value The value to clamp.
	 * @param mixed $min   The minimum bound. Must be less than or equal to `$max`.
	 * @param mixed $max   The maximum bound. Must be greater than or equal to `$min`.
	 * @return mixed The clamped value. Either `$value`, `$min`, or `$max`.
	 *
	 * @throws ValueError               If `$min` is greater than `$max`, or if `$min` or `$max` is NAN, and the ValueError class exists (PHP 8.0+ or polyfilled).
	 * @throws InvalidArgumentException If `$min` is greater than `$max`, or if `$min` or `$max` is NAN, and the ValueError class does not exist (PHP 7.x).
	 *
	 * @phpstan-template TValue
	 * @phpstan-template TMin
	 * @phpstan-template TMax
	 * @phpstan-param TValue $value
	 * @phpstan-param TMin   $min
	 * @phpstan-param TMax   $max
	 * @phpstan-return TValue|TMin|TMax
	 */
	function clamp( $value, $min, $max ) {
		$throw_value_error = static function ( string $message ) {
			// The ValueError class was introduced in PHP 8, so in 7.4 throw InvalidArgumentException instead.
			if ( ! class_exists( 'ValueError', false ) ) {
				throw new InvalidArgumentException( $message );
			}
			throw new ValueError( $message );
		};

		if ( is_float( $min ) && is_nan( $min ) ) {
			$throw_value_error( 'clamp(): Argument #2 ($min) must not be NAN' );
		}

		if ( is_float( $max ) && is_nan( $max ) ) {
			$throw_value_error( 'clamp(): Argument #3 ($max) must not be NAN' );
		}

		if ( $max < $min ) {
			$throw_value_error( 'clamp(): Argument #2 ($min) must be smaller than or equal to argument #3 ($max)' );
		}

		/*
		 * The upper bound is checked before the lower bound to match the order in PHP's
		 * implementation. Comparison in PHP is not transitive when operands of different
		 * types are mixed (for example, comparing against a bool coerces both operands to
		 * bool), so both bounds can compare as exceeded at once, and the first check wins.
		 */
		if ( $value > $max ) {
			return $max;
		}

		if ( $value < $min ) {
			return $min;
		}

		return $value;
	}
}

// IMAGETYPE_AVIF constant is only defined in PHP 8.x or later.
if ( ! defined( 'IMAGETYPE_AVIF' ) ) {
	define( 'IMAGETYPE_AVIF', 19 );
}

// IMG_AVIF constant is only defined in PHP 8.x or later.
if ( ! defined( 'IMG_AVIF' ) ) {
	define( 'IMG_AVIF', IMAGETYPE_AVIF );
}

// IMAGETYPE_HEIF constant is only defined in PHP 8.5 or later.
if ( ! defined( 'IMAGETYPE_HEIF' ) ) {
	define( 'IMAGETYPE_HEIF', 20 );
}