PHP: Roman numeral to int converter

roman_to_int.php

<?php

	/**
	 * Takes a roman numeral input and outputs its integer value.
	 */
	function roman_to_int( $in ) {

		$out = 0;
		$i = 0;
		$prev_map = 0;

		// loop through each char
		while ( isset( $in[$i] ) ) {

			// map our input to a number
			switch ( $in[$i] ) {
				case 'I':
					$map = 1;
					break;

				case 'V':
					$map = 5;
					break;

				case 'X':
					$map = 10;
					break;

				case 'L':
					$map = 50;
					break;

				case 'C':
					$map = 100;
					break;

				case 'D':
					$map = 500;
					break;

				case 'M':
					$map = 1000;
					break;

			}

			// add to out
			$out += $map;

			// is our number greater than the one we had before?
			if ( $map > $prev_map ) {

				// subtract out with the previous int twice
				$out -= 2 * $prev_map;
			}

			// save prev map
			$prev_map = $map;

			// increment $i
			$i++;
		}

		// return the int
		return $out;
		
	}

	echo roman_to_int('II'); // 2
	echo "\n";

	echo roman_to_int('XX'); // 20
	echo "\n";

	echo roman_to_int('VI'); // 6
	echo "\n";

	echo roman_to_int('XI'); // 11
	echo "\n";

	echo roman_to_int('IV'); // 4
	echo "\n";

	echo roman_to_int('XIV'); // 14
	echo "\n";

	echo roman_to_int('XIX'); // 19
	echo "\n";

	echo roman_to_int('XCIX'); // 99
	echo "\n";

	echo roman_to_int('XCV'); // 95
	echo "\n";

	echo roman_to_int('DCCCXC'); // 890
	echo "\n";

	echo roman_to_int('MDCCC'); // 1800
	echo "\n";

	echo roman_to_int('I'); // 1
	echo "\n";

	echo roman_to_int('V'); // 5
	echo "\n";

	echo roman_to_int('X'); // 10
	echo "\n";