Source

File: wp-includes/functions.php

function get_status_header_desc( $code ) {
	global $wp_header_to_desc;

	$code = absint( $code );

	if ( ! isset( $wp_header_to_desc ) ) {
		$wp_header_to_desc = array(
			100 => 'Continue',
			101 => 'Switching Protocols',
			102 => 'Processing',
			103 => 'Early Hints',

			200 => 'OK',
			201 => 'Created',
			202 => 'Accepted',
			203 => 'Non-Authoritative Information',
			204 => 'No Content',
			205 => 'Reset Content',
			206 => 'Partial Content',
			207 => 'Multi-Status',
			226 => 'IM Used',

			300 => 'Multiple Choices',
			301 => 'Moved Permanently',
			302 => 'Found',
			303 => 'See Other',
			304 => 'Not Modified',
			305 => 'Use Proxy',
			306 => 'Reserved',
			307 => 'Temporary Redirect',
			308 => 'Permanent Redirect',

			400 => 'Bad Request',
			401 => 'Unauthorized',
			402 => 'Payment Required',
			403 => 'Forbidden',
			404 => 'Not Found',
			405 => 'Method Not Allowed',
			406 => 'Not Acceptable',
			407 => 'Proxy Authentication Required',
			408 => 'Request Timeout',
			409 => 'Conflict',
			410 => 'Gone',
			411 => 'Length Required',
			412 => 'Precondition Failed',
			413 => 'Request Entity Too Large',
			414 => 'Request-URI Too Long',
			415 => 'Unsupported Media Type',
			416 => 'Requested Range Not Satisfiable',
			417 => 'Expectation Failed',
			418 => 'I\'m a teapot',
			421 => 'Misdirected Request',
			422 => 'Unprocessable Entity',
			423 => 'Locked',
			424 => 'Failed Dependency',
			426 => 'Upgrade Required',
			428 => 'Precondition Required',
			429 => 'Too Many Requests',
			431 => 'Request Header Fields Too Large',
			451 => 'Unavailable For Legal Reasons',

			500 => 'Internal Server Error',
			501 => 'Not Implemented',
			502 => 'Bad Gateway',
			503 => 'Service Unavailable',
			504 => 'Gateway Timeout',
			505 => 'HTTP Version Not Supported',
			506 => 'Variant Also Negotiates',
			507 => 'Insufficient Storage',
			510 => 'Not Extended',
			511 => 'Network Authentication Required',
		);
	}

	if ( isset( $wp_header_to_desc[ $code ] ) ) {
		return $wp_header_to_desc[ $code ];
	} else {
		return '';
	}
}