Convert Integers to Roman Numerals

add_shortcode.php

<? php

add_shortcode( 'roman_copyright', 'roman_copyright_shortcode' );

/* to use the shortcode, insert this into the footer

[roman_copyright before="Copyright " first="2003" after=" Victor M. Font Jr."]

 */

roman_copyright_shortcode.php

<?php

function roman_copyright_shortcode( $atts ) {

	$defaults = [
		'after'     => '',
		'before'    => '',
		'copyright' => '&#x000A9;',
		'first'     => '',
	];
	$atts     = shortcode_atts( $defaults, $atts, 'roman_copyright' );

	$output = $atts['before'] . $atts['copyright'] . '&nbsp;';

	if ( '' !== $atts['first'] && date( 'Y' ) !== $atts['first'] ) {
		$output .= roman_numerals( $atts['first'] ) . '&#x02013;';
	}

	$output .= roman_numerals( date( 'Y' ) ) . $atts['after'];

	return apply_filters( 'roman_copyright_shortcode', $output, $atts );

}

roman_numerals.php

   <?php
   
   function roman_numerals($integer) {

    // Convert the integer into an integer (just to make sure)
    $integer = intval($integer);
    $result = '';

    // Create a lookup array that contains all of the Roman numerals.
    $lookup = array(
       'M' => 1000,
       'CM' => 900,
       'D' => 500,
       'CD' => 400,
       'C' => 100,
       'XC' => 90,
       'L' => 50,
       'XL' => 40,
       'X' => 10,
       'IX' => 9,
       'V' => 5,
       'IV' => 4,
       'I' => 1);

    foreach($lookup as $roman => $value){
        // Determine the number of matches
        $matches = intval($integer/$value);

        // Add the same number of characters to the string
        $result .= str_repeat($roman,$matches);

        // Set the integer to be the remainder of the integer and the value
        $integer = $integer % $value;
    }

    // The Roman numeral should be built, return it
    return $result;
}