元号取得関数（明治〜令和）

get_gengou.php

<?php
function get_gengou( $time, $format = '%s%s' ) {
	list( $y, $m, $d ) = explode( ' ', date( 'Y m d', is_numeric( $time ) ? $time : strtotime( $time ) ) );
	$gengou = '';

	if ( $y < 1868 ) {
		return $gengou;
	}

	if ( $y >= 1868 && $y < 1912 ) {
		$gengou = '明治';

	} else if ( $y >= 1913 && $y < 1926 ) {
		$gengou = '大正';

	} else if ( $y >= 1927 && $y < 1989 ) {
		$gengou = '昭和';

	} else if ( $y >= 1990 && $y < 2019 ) {
		$gengou = '平成';

	} else if ( $y >= 2020 ) {
		$gengou = '令和';

	} else if ( $y == 1912 ) {
		if ( $m < 7 ) {
			$gengou = '明治';

		} else if ( $m > 7 ) {
			$gengou = '大正';

		} else {
			if ( $d <= 29 ) {
				$gengou = '明治';

			} else {
				$gengou = '大正';
			}
		}

	} else if ( $y == 1926 ) {
		if ( $m < 12 ) {
			$gengou = '大正';

		} else {
			if ( $d <= 24 ) {
				$gengou = '大正';

			} else {
				$gengou = '昭和';
			}
		}

	} else if ( $y == 1989 ) {
		if ( $m > 1 ) {
			$gengou = '平成';

		} else {
			if ( $d <= 7 ) {
				$gengou = '昭和';

			} else {
				$gengou = '平成';
			}
		}

	} else if ( $y == 2019 ) {
		if ( $m > 4 ) {
			$gengou = '令和';

		} else {
			$gengou = '平成';
		}
	}

	if ( $gengou === '明治' ) {
		$year = $y - 1867;

	} else if ( $gengou === '大正' ) {
		$year = $y - 1911;

	} else if ( $gengou === '昭和' ) {
		$year = $y - 1925;

	} else if ( $gengou === '平成' ) {
		$year = $y - 1988;

	} else if ( $gengou === '令和' ) {
		$year = $y - 2018;
	}

	if ( $year == 1 ) {
		$year = '元';
	}

	return sprintf( $format, $gengou, $year );
}